Arters, that was before the numbers were ever revealed. For the A310 or B767 it looked to be around 24 as you say.
But then someone evidently thought that with an aircraft carrying 110 instead of 73 tonnes, i.e. if you assume a 10 tonne t/o, reserve, approach etc total, then perhaps 24 x (63/100) = 15 might work. Less crews to pay, less ac to maintain, less hangarage needed....
Only one slight flaw with that plan - 'twas bolleaux.
One u/s jet out of 24 sitting on the line = 96% still available
One u/s jet out of 15 sitting on the line = 93% still available.
But since when did the RAF ever have only 1 u/s jet?
And, most important of all, it is quite common not to need 110 tonnes in any one place, but 55 in two places.....