Yeah, the good-ole 'TE' method. try it, try it, try it...

Different planes will yeild differnt results with the same prop. Also, one brand '11x7' prop will not perfom the same as the next brand '11x7' prop (nor will it fit the spinner!!)

Air quality on the day will affect the prop's performance, as well as altitude, barometric pressure, humidity, etc...

There are many, many factors to choosing a prop. As long as you are in the 'ballpark' for the reccomended prop size, all you can do is try..

If you search this site for 'prop size' and 'which prop to go faster' you'll get some other explanaitions (not the exact search string, but along those lines.. There are many threads about choosing props...)