Cab, the performance altitude number is a new calculation Computec uses that is considered more accurate than the older Density Altitude. Performance altitude on Saturday at BIR in MN was 2427 ft. BUT the density Altitude, the number most of us are familier with, was 3275 feet! So i am pretty happy knowing i ran what i did and fat as it was tuned.
Computec tech guy said to go by the hp correction factor for best et predictions. For that last run it was 1.0884. Which with sea level conditions would lower my et to 8.704 from 8.854