RE: SYSSA 30CC GAS MADE IN USA
I use that rule to put myself in the ball park too. I'm running an 18x6 on a fairly heavy plane and getting pretty good performance (the engines loves rpm's!). So using that rule and going down in diameter, you would get 17x8, 16x10, then 15x12? So a little less diameter would let you keep that pitch, but from a vertical performance standpoint, the 16x10 might work better?