Precision is the bane of working with pitch and other grinding chores for the beginner. Don't worry about it. The more important thing is to be sure that the lap is "pressed" often. I usually leave the mirror on the lap and if the lap dries out, a bit of water squirted between the mirror and tool will loosen the mirror. Also, don't be afraid when the tool starts squeaking - it's an indication of it working hard on a zone. I might also mention using some of that micromesh (it's that netting that the wife uses for sewing) - it works and works well. What it does is give more edges for the rouge to work with.