ok.. so after i fly long flights in fs2002 and i'm getting rdy to set up for APR, i usually use the landing approach spds given on the kneeboard, and take off about 8-10 KNTS per 100,000 LBS of fuel consumed. it works usually....but how would a real-world pilot calculate this? i'm sure they don't have too much room for error. and it can't be from trial and error