Now, segmenting with skull bone, that is getting "cheeky" and not easy. You may get better results in using the bone with a darker wood, for contrast...!
Yeah, Brooks suggestion is very good, so it is the caliper suggestion from Hunter-27...!:wink:
You can make a jig to help you out, no doubt but, I found one of the easiest ways to solve the problem of a worn bush or a slightly undersized one, even if new, is to rap 1 or 2 layers of masking tape around the bush, flash to the "good" edge.
You just have to remember to not overlap the tape if you only want 1 layer of tape. You can make it safer and add 2 layer of tape, 1 to guide you with the barrel size (rough turn) and remove 1 layer to guide you for the hand sanding.
You can use different colours for the layers and you can always replace the tape if it gets damaged before you're ready...!:biggrin:
It doesn't take more than a few seconds to rap some tape to the bush(s)...!
Good luck...!