How to set offset tool length?
CNC Cookbook: Tool Length Offset and Tool Data Management Part 1
Select the length offset of the reference tool. Lower it until it's zeroed on the top of the part, then set Z to where you want the top of the part to be. It's common to have Z=0 at the top of a part, so any positive Z is inside the part. Z=height of the part is also common.
