As far as I understand, all JP Calk really has is the impeller cut RPM/HP charts electronically accessed. That and a calculation of speed relative to RPM per cut. It should be as accurate as any of the impeller charts. I'm waiting to see LVJetboy chime in on it tho as it is his program.
It seems to be right on in my case. My engine is turning about 350 HP accordign to Desktop Dyno ( I have tried to be as accurate as possible with it) and the RPM I am spinning my berk "A" fits 349HP, 4800 RPM. I will see what next spring brings as my headers should put me right at 400HP and 5,000 RPM.