Depends on what tools you have available. Assuming you don't have
things as surface tables and height gages, or even a good
caliper..........
I would use a divider or compass. Just set it at your best guess at
half the diameter, and scribe arcs from the outside. If the arcs from
opposite sides don't intersect, increase the size of the arc. If three
or four arcs define an area that is too big, decrease the size of the
arc. Should be able to get to your 1/100th of an inch, but not much
closer.

Dan