I played with tool offsets in Mach 3 over the weekend, it has been awhile. It looks like experimenting from the diagnostics screen give you the best information about what is going on. As I thought the tool offset actually occurs when the next Z move is commanded not when you call G43 H(tool offset#) Also the manual is a little confusing, seeming to say that offsets can't be negative. But reading closely the H number can't be negative but the offset can.