I drill 3 15mm deep 12mm wide holes on each cheek. One in the middle and one on each side of the two holes there. Not exact science, but gets me close enough. I'll go all the way through for 100cc's. You need to counter sink the edges to prevent cracking. You can fill the holes with jb weld if you know what you are doing, helps bring back the c.c. compression.
Edit: I probably should have said I run 66's. The three hole 15mm deep is about 40-45% balance factor for a 66. All the way through is about 40% for a 100cc. I would probably only do about 10mm for a 50cc. I usually don't like to run my bikes over 7k rpms( I am trying to make them last, and take care of them, I love my bikes). All the cranks I ever drilled, run smooth and almost no buzzing feeling till you get way up there in the rpms. Really makes a huge difference.
