I'm woking n a jig,Tim

this requires 4 metric 6" rulers affixed to a tin/ally plate in a manner they slide in and out.

The idea is the tin plate has an opening to fitove the jaws and the rulers slide down onto the jaws and using the rulers increments as a guide the jaws can be adjusted for centering on all four.

Sounds complicated and tedious but its not in my mind,at the drawing stage at the mo