Hey,
where is the diagonal mentioned? The Official Onefinity CNC Assembly Video does mention no diagonal, it just fixes the two front feet flushing the front edge of the table and then, with X Axis mounted, slides the X Rail backwards. As Nick stated, this is unreliable for obtaining an accurate right angle. I would consider 0.1″ (2,54 mm) as a total no-go.
I would never assemble the machine without diagonal measurement for the screw holes to be drilled.
Thanks,