Let's try this again.

CONVERSION RATE OF DBS ALTITUDE (S$4,000 spend/month)
Assume you put S$4,000/month on a DBS Altitude. Each month you get:
First S$1,000 = 1,200 SQ miles
Next S$3,000 = 4,800 SQ miles

S$4,000 per month / 6,000 SQ miles = estimated S$0.6667 per SQ mile

BUSINESS CLASS REBATE (New York)
I am looking up SIN-JFK business class round trips for "comparable" airlines for August 6 - August 13 (National Day weekend). I entered this into airfares.com.sg.

We get Cathay for S$7,851.70, with Turkish at S$7,232.70. On SQ, it's 114,750 miles on Saver or S$10,200, plus taxes and surcharges of S$811.10. To redeem, you need 114,750 SQ miles plus S$811.10.

To get 114,750 SQ miles with a monthly spend of S$4,000, you need to spend S$76,500. S$7,040.6 (Cathay price less surcharges/taxes) divided by S$76,500 is 9.2034%. S$6,421.60 (Turkish price less surcharges/taxes) divided by S$76,500 is 8.3942%. Pretty good.

BUSINESS CLASS REBATE (Frankfurt)
For the same dates for Frankfurt (there is no Saver for Paris business so we are not using Paris), a search for business flights returns Emirates, at S$6,367.50 (with Thai Air at S$4,285.50). On SQ, it's 93,500 miles on Saver or S$6,500, plus taxes and surcharges of S$639.

To get 93,500 miles with a monthly spend of S$4,000, you need to spend S$62,333.33. S$5,728.50 (Emirates price less surcharges/taxes) divided by S$62,333.33 is 9.1901%. S$3,646.50 (Thai price less surcharges/taxes) divided by S$62,333.33 is 5.8500%. Still not bad but it may seem much worse than the New York flight, depending on how you compare a Thai Airways business class seat to an SQ one.

ECONOMY CLASS REBATE (New York)
For the same dates for New York, a search for economy flights also returns Cathay, at S$2,286.70. On SQ, it's 68,000 miles on Saver or S$7,400, plus taxes and surcharges of S$780.70.

To get 68,000 miles with a monthly spend of S$4,000, you need to spend S$45,333.33. S$1,506 (Cathay price less surcharges/taxes) divided by S$45,333.33 is 3.3221%. Much worse than business, but still not bad.

ECONOMY CLASS REBATE (Frankfurt)
For the same dates for Frankfurt, a search for economy flights returns Emirates, at S$1,522.50 (with Malaysian Airlines at S$1,149.30). On SQ, it's 59,500 miles on Saver or S$1,710, plus taxes and surcharges of S$593.60.

To get 59,500 miles with a monthly spend of S$4,000, you need to spend S$39,666.67. S$928.90 (Emirates price less surcharges/taxes) divided by S$39,666.67 is 2.3418%. S$555.70 (Malaysian price less surcharges/taxes) divided by S$39,666.67 is 1.4009%. Again, much worse than business but still better than the roughly 0.5% you get back from redeeming other credit card rewards.

COMPARISON TO OTHER REBATES
Obviously, redeeming business class tickets is great value for money and will be more than the highest cash back reward (5% for Standard Chartered's Manhattan, with some very cumbersome restrictions). This assumes you are at least going to Europe, as there are so many budget alternatives for shorter flights that drive down your conversion ratio for these.

If we take the "worst case" benchmark, 59,500 SQ miles is equal to S$555.70, or you may as well book a Malaysian Air ticket. That is S$1 = 107.07 miles or 1 SQ mile = 0.93 cents.

If we consider at least Emirates to be equal to an SQ seat, 59,500 SQ miles is equal to S$928.90. That is S$1 = 64.05 miles or 1 SQ mile = 1.56 cents.

If you will be redeeming for business class only, 93,500 SQ miles is equal to S$3,646.50. That is S$1 = 25.64 miles or 1 SQ mile = 3.90 cents.

If you will be redeeming for business class only and consider only Cathay to be equal to an SQ seat, 93,500 SQ miles is equal to S$5,728.50. That is S$1 = 16.32 miles or 1 SQ mile = 6.13 cents.

What does this all mean?

Simply, if you have a choice between air miles and another deal, take the other deal if the cost per SQ mile is higher than the above. With some common sense regarding the business class conversions.

Example 1
For example, should you renew a Citi Premiermiles card for about S$180 (getting 10,000 free SQ miles) or have the fee waived? S$180 divided by 10,000 SQ miles is 1.8 cents per SQ mile.

This seems to say, you should only get the free 10,000 SQ miles if you are redeeming business tickets; if you do not spend enough to do this, save your money to pay for taxes and surcharges. However, note that you are still losing S$180. As you would never save up to actually buy a business class ticket using cash (as opposed to using miles), there is just no point to buying miles (or if you do plan to buy miles, use the economy ticket benchmarks).

Consider that otherwise, you would actually pay a small store a 2% to 5% credit card surcharge just to generate miles to redeem for business class tickets. That makes no sense.

Example 2
The highest paying rewards card in town is the American Express Platinum Reserve Card, which gives 20x rewards at (a small number of) American Express Platinum Partners. How much is this really worth?

S$1 = 20 rewards points = (5/8 x 20 SQ miles) = 12.5 SQ miles.

12.5 SQ miles x 0.93 cents per mile is worth S$0.11625 per dollar spent.

Following the worst case discount, buying at 20x rewards at a platinum partner is about an 11.625% discount. This goes up to a 19.50% discount if you use Emirates as an economy ticket benchmark, as above. Pretty good (again, in the small number of platinum partners, and only if you qualify for the minimum S$150,000 per year income level and pay the large annual fee; you do to get certain other Amex Platinum discounts).

Note, you need some common sense for this if you plan to redeem for business class tickets. Unless you actually plan to purchase business class tickets for cash if you do not have enough miles, 20x rewards is not giving you a 48.75% or a 76.625% discount at a platinum partner. Otherwise, you may as well go into Hour Glass and buy the most expensive Patek Philippe they have at full price and tell yourself you are getting an incredible deal through air miles.

Don't lose sight of the ultimate point that you generate air miles from what you would normally spend; you are not supposed to pay money or buy something you otherwise would not buy just to get air miles as that cannot logically be a good deal.

You can repeat this thought process for every other 10x and 5x rewards program out there.

Example 3
For a simpler example, should you pay for a meal using your DBS Altitude if you can use another card and get a 10% discount? The easy answer is you should always take the discount as you will rarely get up to 10% of your money back by redeeming air miles.

Use the savings to pay for taxes and surcharges.

CONCLUSION
I will assume you can get a rough 2% rebate by converting rewards points to air miles.

As you can see, this is very convoluted and credit card companies make one think they are getting a lot more than they actually are.

I wanted to run this again as the rebate will get lower the closer the destination, and New York is probably not a fair benchmark. It is also not worth redeeming SQ miles for short flights that are covered by budget airlines.

Three things. First, is redemption for business class flights really the best option? You need to spend about 50% more on your card to get a business class flight. This means you can book only two tickets per year instead of three. If you spend (or reimburse) enough on your card to take all the flights you want anyway or you are simply too tired after a long flight in economy, then no debate there.

Second, is it worth getting a rebate card such as the Standard Chartered Manhattan, which gives a cash rebate of 5% per month you spend at least S$3,000, up to S$200 every three months. It looks like a better option to get a Maybank Horizon to improve your rebate rate if you have a lot of overseas spending or reimbursement, or another specialized card (ex. groceries).

The Manhattan's rebate rules are too much of a hassle to monitor for the small incremental rebate. At most, you get about S$480 per year (assuming you use only a DBS Altitude) and only if you can time all your purchases properly (which means, spend exactly S$4,000 in one month every three months and nothing at other times).

Of course, if you know for sure that you are spending S$3,000 to 4,000 in one month (buying a new laptop or planning a major shopping spree, or throwing a wedding banquet after you got a Manhattan card for your fiancee, father, mother, brother, sister and dog and split up the bill), then take the 5% in cash. Forget air miles (again, unless you actually buy business class tickets in cash).

Third, the above rough benchmarks assume you are using only one card. If you use more than one, remember to add the conversion cost almost all the cards have, whether annually (DBS Altitude, Amex Krisflyer) or per conversion (Citi, Amex Platinum). The conversion cost makes it more expensive to use multiple cards for the best rewards rate per transaction, as your miles will be spread over many cards and may have to pay a conversion fee for each card.

Does everything above look right to you guys?