Not at all sure how all this works....
I remember we had to go down this line of digging at Boscombe for some project we were working on. You know how sometimes thinking too much takes you down some very unexpected roads. Luckly some body managed to pull out some old information from somewhere which showed that it was basically arrived at after a statistical analysis of a fairly widely scattered sample of data involving quite a wide range of aircraft types. Very interesting to read everyones thoughts, going to bed now as it is late.