I find that prop selection is very inexact. Many factors change your prop needs. Examples are one engine having more power than another. One engine makes it's peak power at one rpm and another at a different rpm. Flying styles make a difference. Even the fuel makes a big difference as well as how far above sea level you are.

Having said that you should have a selection of props. Look in your manual and find the reccomended sizes. Buy a range of them on either side of the reccomended. And remember this simple addage:

Diameter=Power Pitch=Speed

You just need to use a few on your combination og engine and airframe until you find a combination that pleases you.

Mark Shuman