Osborne maneuver

[Toolmaker51]

Marv's ability to illustrate an issue, and describe solutions is uncanny, and typifies highest level of mentoring. To write software and offer it gratis, is beyond generosity. Today alone, there've been 2-3 programs already suiting what I have to do regularly. We are literally fortunate to be his students. Not only in personal benefit, but to emulate and broaden the ranks of those we associate via other means. And that's good for all of us.

Or in for-profit situation, use stock a little oversize. 4 hits X +/- Y +/- will do the trick. Knowing your backlash makes it partly, but reasonably accurate.
With a DRO, something rigid i.e. endmill shank or dowel works well. I don't hit one side [say X-] then 6 o'clock one of the Y's; going all the way across first. Divide the reading x.5, and transit to the axis thus calculated and repeat touching off two sides of second axis. Done. A corner, is 'found' the same way, but 2 hits + half contact diameter will give the coordinates. An actual edgefinder is more accurate, but requires a toolchange.
Round, square no matter; really it's a CMM variety of size determination, by subtracting stylus diameter.