If you have a way to check the corners for focus, then just unscrew the lens and test. If you know the pitch of the threads, you know how far out the lens comes with each revolution. I did some film tests with my Angulon 210 at various front/rear cell spacing. but went back to the factory specs. With that lens I it appeared that the focal "plane" was not so much a dish shape but more like a sinusoidal pattern of concentric rings like a drop of water in a puddle. As I got the very far corners to match the center, other areas of altered near or far focus would crop up at intermediate distances from the center. The factory spacing gave the best compromise.