I suppose what someone needs to do is to develop an APP where the person is told to measure the distance from the drive axle to the fork face; enter the rated load distance and rated load; and then enter the load distance in question.
I've checked Dan's rule for the 25 "5000 lb" forklift models I have data on and it never over-estimates by more than 1%, and is conservative for load distances of 34" or more. On that basis it's fine to use for that size of forklift.
However for 3000 lb forklifts it underestimates capacity by 20% and for 6000 lb forklifts it overestimates capacity by 9% at 36"
A better rule for forklifts up to 7000 lb would be to reduce the load by 1/50th of the rated load per 1" increase in load distance. For 3000 lb forklift 60 lb; for 4000 lb forklift 80 lb; for 6000 lb forklift 120 lb and for 7000 lb forklift 140 lb