Lay it all out after it's chucked up in the lathe. Scribe the circle by
using the crossfeed micrometer to set the radius as you described, then
set the scribing tool as near as you can to vertical center. Use the
division pin stop in the lathe headstock to lock the chuck and cross
scribe with the crossfeed. Turn the chuck 180 degrees and lock again.
Repeat the scribe mark to confirm that it is centered.
Rotate 90 degrees and repeat for the other two holes.
This procedure works for any bolt circle that is divisible by the
number of stop pin holes in the headstock, usually 64.
Bugs