"prop factor" is a good quick guestimate, but it does break down when you start moving more than 2-3" in diameter away. Also, I've seen a couple of different formulas, Kennyroy's being one of them, and plenty of people swear by their formula. It's an inexact science at best.

That chart posted looks pretty good, better than some that I've seen. Though I will say that I went and checked some engine/prop combinations I use, and I found them outside that chart's range. Not a big deal, of course, the chart is a good place for a start, and you won't blow your engine up with it. The chart also doesn't seem to show some prop sizes that you might encounter, such as the ones in the 12x4 range the 3D guys like to run.

Also, as I think someone else mentioned, differnet brands of prop in the same size will often pull very differently in the air. So sometimes you can change performace just by changing prop brands.