Larry,

Understood. But all other measurements are accurate except A to B. If I adjust for A to B then all other measurement points are affected.

These are accurate right now:
A to C
B to D
A to D
B to C
C to D

I don't understand how A to B can be inaccurate given all other points are accurate to a precision measurement (a hole punch) using a tram gauge. It's mind boggling. I'll try a few variations using your principle and report back.

Thanks!

Wade