Something for folks to think about when calibrating a machine: It’s only as good as the tool you use.
A framing square might be a good route, but any amount you’re off gets exaggerated the bigger your part is from the square.
Using a tape measure is fine for checking machine square because even if the tape is inaccurate if you’re pulling diagonals all you need is the same reading across each diagional.
If you’re calibrating length you might want to consider this:
There are actually different “classes” of tape measures. The ones I’m sure all of us have are class 2 or worse.
Class ones like a Tajima: https://www.amazon.com/Tajima-H1550Mw-Hi-Lock-Measure-4781540/dp/B00UC318M4?source=ps-sl-shoppingads-lpcontext&ref_=fplfs&smid=AP3VA1GJZM3EQ&th=1
Or a Starret: Starrett Tape Measure – 35 Ft - Certified – Atema are likely more accurate.
All that being said: Our machines are setup and tuned to our rack drives, gearing and steps so you shouldn’t have to do this unless there is something odd going on.