Bit perverse for me actually. Bear in mind miles have HK$0.078 value when used to pay fees:

For my nephew's end of HSC (DSE/A-Levels) trip in 2017 (i.e. pre-22 June 2018 changes), my brother's family burned 840,000 of my Asia Miles to do this 3-week trip in Europe starting in Italy and ending in Spain (starting in Australia, all Standard J). My miles cost was $0.06 back then https://www.hongkongcard.com/forum/show/22044 #5 so I effectively paid $50,400 out of pocket.
I was paying fees out of miles back then.

For my niece's end of HSC trip this year, I will burn 1,494,000 AM (still in Business, but I'm being shafted into almost redeeming exclusively in Choice J) for them to do 3 weeks in Canada and US. My miles cost has gone up from 0.06 to 0.08 (I poured 30,000 Rewardcash (each with $1 intrinsic value) into 375,000 Asia Miles via Moneyback conversion - i.e. $0.08 per AM out of lack of competing offers for cheap miles). If the trip goes ahead I'll be $125,120 worth of miles out of pocket - I did consider whether to scrap the redemptions and make them fly PE revenue to Europe.
But that'll give them the perfect excuse to evade Hong Kong and I will have the elders to answer for for this.
I paid all my fees in cash last year.

I think with these two large redemptions out of the way my need for miles has come down considerably. I guess I can stick to not more than $0.078 for paying for miles. But I will still pay my fees in cash.