On higher rpm engines I've always leaned toward a slight overbalance. Usually somewhere around 52 to 54% "bobweight". As Terry said, it's more trial and error than science though. One reason for that is, how much fuel/oil is hanging on the piston whilst running? Answer, it's never the same and at different rpm's it's constantly changing affecting the balance just a tad. Just one of the many reasons why experimentation is necessary to find a balance that works best for your particular setup. Another is the harmonic frequency of the airframe/mount. It goes on and on and on.