If you select a propeller such that your O.S. FS-91 II will turn in the mid- to high-9,000's when leaned to just rich of peak RPM, you'll be running right in the best power band. This is where we tend to run our engines. Any prop that will give you that RPM and fly your model as desired is the "correct" propeller. Any idle 2,500 RPM or lower is considered "good".