Just to give you an idea of how accurate the Airboy spreadsheet can be if used correctly:
Airboy results: 308/323
Went to a Mustangdyne chassis dyno three days later. My results: 308/323
I posted logs of that Airboy 308/323 run in my thread.
Yes, you can cheat with the spreadsheet. The same goes for tuners. They can cheat with their dyno's. Me personally, I probably will never go to a chassis dyno again. I made the Airboy/Trap speed thread for a reason. Once I get to the track, I will post what I made on the Airboy spreadsheet and post my trap speeds. Trap speeds are what really tells you how much you are making.
My personal goal is to have a reliable vehicle that will trap 118+. I personally won't be "happy" until that is accomplished.