Correct math for user defined parameters

I?m trying to get the formula correct to get the input shaft rpm of my t56. (((Gear ratio*336)*mph)/(tire height in inches))*current trans gear ratio or in hp tuners I have ((1444.8*[sens.20.Imp])/26)*2.66

2.66 first gear
26 inch tall tire
4.30 gear * 336= 1444.8

My problem is that I?ve tried this out on my 99 trans am which is an auto, my engine rpm and input shaft speed were not the same as the what my formula gave out. I tested this in 4th gear with converter locked.

My number stayed around 30 to 50 rpm lower most of the time. At one point my user defined deal went up to a few more rpm than the engine rpm which doesn?t make sense.

Can anyone tell me what the formula hp tuner uses to get input shaft rpm.

I?m using an old version 2.22 and cannot upgrade for a specific reason.

Bump, really could use this info in getting it setup right