I have the WinPep (Dynojet) program. For info purposes, my 94 does approximately 400 rwhp. I can vary that by a range of approximately 15 hp by selecting one of five correction formats. With my car the STD, gives the highest and the UNCORRECTED, the lowest. That will obviously vary relative to ambient conditions. Once the data file is generated, the output data can be manipulated after the fact with the program.
Another issue - I have seen a multitude of dyno posting, however, most do not have the a/f ratio included. Request the operator to use the multiple graph option so that hp, torque and a/f are all included on one sheet. It is far more helpful than a second sheet that includes the a/f ratio.
Dyno's are tough on half shafts. Don't allow the operator to start the pull below 3000 rpm's and don't allow them to go to far past 5000. There is no sense, since, your maximum ratings are typically within those ranges. All you are looking for is an increase in the curve and that is obvious within the stated range. The torque reading can be maxed by starting early, however, you are taking a chance. These concerns are exacerbated with modified cars.
Also - shift easy up to fourth and the pull is done in four gear. The flywheel hp and torque is rear wheel/.85. The older the car gets the smaller the denominator gets. An older car starts to suffer higher losses in the drive train, hence, it could be rear wheel/.83