I'm not sure I'd trust your method of measuring as it won't be very accurate, just the length of the arm its self will deflect a fair bit. Unless you have a flat reference surface to measure from then you'll never truly know for sure how far your measurements are off.

To me you have 2 choices, Fit the rails as it is and see how it cuts and then shim out any issues. Or just bite the bullet and go with epoxy.

What size machine and what's the main usage.?