Never did it for real (the "cowshed" doesn't count) but using IAS in the diagram above doesn't seem quite right.
TAS might be ok, but you could be caught out with the wind.
Ground Speed (GS) would be better methinks...
That's the one thing I could not remember... It was 12 years and counting ago!
It might be GS.....
BEX; changed diagram to reflect GS. Thanks!