I finally got to ask my mentor about this. He said his best method is to lay a granite surface plate on the bed, and indicate in a circle with a collet-mounted horizontal beam. I guess the longer the beam the more accurate, until the weight and flex of the beam overcomes accuracy gained by distance.

The discussion of those techniques tell us how to get it as square as possible, but do not really address the issue that is bothering me, which is how much time I lose setting it square after moving the head to mill at some angle.

What I had been doing after using the head to mill angles is to run the quill down, place a square on the table next to the column, and eyeball it to close to square to begin the process. Prior to my original post. I experimented with indicating the column while moving the table, which turns out to be a less than optimal solution, rather than a shortcut.

The sort of work I am doing,or at least how I am doing it, demands as much accuracy as I can get. In a perfect world, I would have been able to afford machines more suitable to obtaining higher tolerances.

I do like the ability to tilt the head of the mill. It seems to me that in designing a machine with such a capability, there would be a provision to quickly and painlessly return and lock the head to square on both axes.

Certainly it seemed worth the effort to post the question here, before going to the trouble of engineering a solution to my problem, only to learn later that an effective and elegant solution is already out there.