There are a few prop load formulas but they do not reflect RPM you will get out of an engine. I use the formula diameter^3 x pitch. That gives a 10/6 a load of 6000. You can use this for trying different sizes of props but maintain the same RPM. If you run a 10/6 and want a larger prop then a 9/8 and a 11/5 would be the closest loads.