ORIGINAL: YOADRIAN
I once was told that one inch less for each additional blade (example: 22x10 2 blade would then go to a 21x10 3 blade any merit to this formula?
Yes...knocking 1-2" off "total" tends to work decently.
As an example, every G62 I've owned likes the Xoar 23x8 prop. When i went to a 3-blade on my big Pitts, I stuck with the same company (Xoar) and went with a 22x8, which gave me similar performance, SLIGHTLY (< 100) fewer RPM's. A 20x10 gave me nearly exactly the same RPM, but slightly less vertical, as you'd expect.
I've seen similar results converting an OS FS70 to 3-blade, and on a friend's DA-50. Up and down the range, taking 1-2 total inches off the 2-bladed prop (by total, I mean combination of pitch and diameter) seems to produce similar engine performance on a 3-blade.