I recently had opportunity to work on squaring a 4'x8' table from a reputable manufacturer. It appears that the machine was never, from the manufacturer setup, very square to begin with. The owner doesn't use it for very exacting purposes so probably never noticed. But I expected that it would be square from the initial setup to within a few thousandths. The thing is, when speaking with tech support from the manufacturer, they suggest using a tape measure to check large square diagonals. I can't see how you can get it square into the single digit thousandths with a tape measure and simple marks on the spoil board. Am I expecting too much? Is it common to be 0.020" or more out of square (offset between one side of X vs the other) on a commercial grade machine of this size?