If you want to use the measurements to build and test a camera back, I'd try to get a new or like-new Toyo holder. They seem to be made to tighter tolerances, and they generally have good flatness and keep the film square in the holder (some have more play, often due to wear, so the sheet can become crooked in the holder). Then if you want to shoot with other holders, at least you'll know the back was made with one that should be in tolerance.