In trying to tune gas engines for general sport flying, I have been trying to find anywhere a definite instruction rather than the old 'back off a couple of 100 rpm's', as to how much should the top end be reduced from it's max rpm before flying to guard against lean running.

I have heard some folks say, reduce by 100 rpm, others say 2 or 300, and another faction advocates to richen the high speed needle a 1/4 turn.

Anybody willing to take the plunge to be more specific on this, or is the variance of such that one can't be specific as each make and size of engine requires a different setting.