I don't see this conversation as being too uptight. My take, with zero frames under my belt but a fair amount of mech. engineering experience (even some that's pertinent to this discussion ), would be to set what matters as the datum. The axle centers matter in real world use far more than how level the top tube is. Accept that the metal will not match the drawing 100% and allow the fudge factor to stack up where it doesn't affect the handling or location of anything critical. Axle centers directly affect the contact patches.