I work for Vermont Photonics Technologies, we wrote some software for doing this a few years ago. The software is called SurfaceView and is specifically meant for using the Union Jack or Moody Method for calibrating surface plates. More info. can be found on our website here:
SurfaceView
There is a free demo version of the software, it is fully functional except that you cannot save data.
If you are just measuring a couple plates the demo should be sufficient but if you are measuring plates commercially you may want to look into purchasing the full version of the software.
On the same page in the link above there is a link to the article J.C. Moody wrote explaining his method for measuring surface plates as well as a link to the Federal specification GGG-P-463c on how graded surface plates are supposed to be measured.
This software is still being actively worked on and updated so feel free to give suggestions, complaints, annoyances etc. and we will see what we can do.
Hope this helps
--Aidan