Hey Ziggy,
you understood well that you can do this with the geometrical method that I mentioned in a posting above, but unlike I suggested here earlier (with the little raspis), later I came to the conclusion not to use the base of the Onefinity’s feet as measuring points to measure the diagonals but rather the points where the rails go into the Y anodized aluminium blocks. The rails are adjustable within their pockets in the aluminum blocks in order to adjust the tilt between upper and lower rail, and therefore they are not always in a predetermined position relative to the corners of the feet. And it’s the rails where the gantries run, not the feet – so better measure on the rails.
Using Starrett things is always a joy, but you can also use the simpler Bar Gauge Heads that I recommend with wooden sticks with sharpened edge, like in the images shown in this item description at Lee Valley website:
Also as Tom mentioned here, the distance between the two blocks on one side can be different on left and right side. And the equal-diagonals method only works in a rectangle, not in a trapezoid, so first measurement should be whether y=y’! The good thing is, this can also be done with the same Bar Gauge method!