Hey Frank,
after having attached the machine to a machine base (e.g. table top, anti-torsion box etc.) like described in the setup manual (hereby using the X gantry once on front and then on rear position to ensure the distance between left and right rails), for adjusting rectangularity (“squareness”) of the machine, it is enough to loosen two of the four machine’s feet, and also slightly loosen the bolts that hold the X gantry on top of the Y carriages, and make the parallelogram a rectangle by ensuring identical diagonals with a bar gauge. No need to measure anything. You can also use the bar gauge to first check whether the Y rails have same length.
You put the bar gauge ends exactly where the chrome-plated, hardened steel hollow shafts enter the black anodized aluminium extrusion feet.
Before surfacing the wasteboard, it is important to ensure the machine is rectangular (“squared”) (bar gauge) and 2. coplanar (“not twisted”) (fishing line method). If the latter is not adjusted, all workpieces (including the surfaced wasteboard) will remain twisted.
I think it would be nice if there was an adjustment mechanism for coplanarity (like shown here), as well as one for trimming the Z axis perpendicularity, like found on industrial machines. At least the QCW Frame owners can adjust coplanarity with the now included Any Surface Leveling System, but the QCW Frame has other disadvantages.