I’ll do this and report back.
The ‘workarea’ sheet isn’t necessarily exactly centered between the motors, or exactly square to them. Errors in that part of construction would introduce error in the calibration. Better to use the motors to eliminate those construction vagaries by having the motors pay out fixed amounts of chain.
It might be hard to be accurate centering a router bit over a snapped location. What we really want are two known points on the vertical line centered between the motors. The error scales with the distance between the cuts, so using an upper and lower cut would double the error and make an accurate measurement easier. The check cuts could be in the y direction, or in the x direction, but slightly longer to differentiate them from the first set. It may well be that we can do the necessary calculation from one set of cuts.