No the calc I use is hp to the ground - i used the DLM calc on his website... its proven quite good before.
Most of those computer programs are good for E.T. calculations, but bad for trap speed calculations.
At least where Vipers are concerned.
I think that it has to do with the Viper's power delivery. Where a stock Gen II Viper will run 12.2 @118MPH, other cars will run more like 12.2 @108MPH. And the calculators are probably built off of a model of general performance stats.
With Motor Trend's last test times of a stock SRT-10 Viper
11.77 @123.7MPH put into the DLM calculator (at 3550lbs), it figures around 431RWHP for the E.T.(which is about right), but it calculates that the car would need 525RWHP to make a 123.7MPH traps, which is over 600 crank HP.
-Dean.