"Dave Garnett" wrote in message
...
snip--

Firstly, put the screws in and then tighten up a lock nut to remove the
wobble. Then do your thing with the micrometer/calipers.

That's a poor way to determine holt circles due to the very common condition
of bolt heads not being concentric with the pitch diameter of the bolts or
screws in question. It's far more reliable to work from the minor diameter
of the tapped hole.

Harold