ORIGINAL: carrellh
Every engine and airplane is different so the ''rules'' are nothing more than a starting point.
The reality is: Three blade prop selection is limited. Your decision likely will be based more on availability than optimum performance.
Zinger says 'keep the pitch' and 'drop the diameter'
http://www.zingerpropeller.com/_2_blade_.htm
Good point Carr. I did find a better selection from Graupner {spelling} but it's almost always mail order. They are as tough as MAS too.