one for the math gurus.....Squaring an Axis

Setting up an Axis to be truly square on a CNC machine has always been a
challenge. It obvioulsy gets a little easier if one is fortunate enough
to have a friend or a day job where you can borrow thier precision
squares and straight edges.

Them babies are big bucks, and I used to be able to borrow one from
where I used to work, but,.....

Obviously, a "circle" can't be cut into a true circle if an axis is
skewed, so,

Is there a mathematical equation where one could:

Engrave a "Dial" with degrees, cut it out into a circle, and then
measure and "map" its actual circular dimensions.

With the above info, could one then take the dimensional "width" of the
Gantry ( based on the mounting points or "fulcrum" point as one end is
adjusted), and calculate the amount of adjustment that would be
necessary to *True up* the Circle ??

I'ts all numbers, so there must be a way to do it. I never came across
such an equation. I doubt the diameter of such a circle would have much
affect, only the differences in its trueness.

Framing Squares really don't cut it, and I'm curious how others go about
this with the least amount of fiddling around.....

Thanks,
Chris L