Correct, but your calculation goes the wrong way.

Sixth Freedom was calculating the value of a BA mile by comparing a revenue CW ticket with an MFU. For the purposes of that calculation, it costs more than 25,000 miles because the CW revenue ticket earns more miles than the WTP ticket - only about 25% of the base mileage though, which for a roundtrip to North America is very roughly 2,000 miles. So he needs to add 2,000 miles, not subtract 8,000, i.e. the value of each miles is the money saved (£1,000 in his example) divided by 27k.

I actually don't think WTP->CW MFUs are too cheap - at least not by very much. If one restricted MFUs to E and W fares and disallowed them for T fares, as per Hiddy's suggestion, you're looking at leisure passenger to New York facing the choice between an E-class return at £1580 + 25k miles (if they can find availability) versus the typical somewhat inflated sale I-fares of £1480 + 0 miles, with much wider availability. That makes a mile worth roughly minus 0.4p, if you're lucky enough to be able to find a suitable flight to use them on.

Sixth Freedom's rough calculations show an MFU is worth twice as much to the leisure passenger as a straight redemption. I don't doubt this, but obviously an Amex 2-4-1 (which is pretty easy to come by - I have two knocking about in my account and would have about eight if I were allowed to earn more than one a year) equalizes that. And a straight redemption is fully flex which is worth something even to the leisure passenger.