Hi c0depr1sm,
I think that with simple calliper modification we can measure position of bit within 0.2mm precision - take a look at my picture and you will fill the dots.
I dive deeper into firmware and Ground Control code but I cannot figure it out where do you record number of pulses for given gcode command (I am lousy coder 
). When I discussed with my neighbour, my approach to encompass all non linearities of Maslow into calibration surface, Greg (his name), pointed to me that we do not have to measure length of chain - we have motor with encoder and all calculations ends up with the number of pulses to move router bit anyhow. My L = f(x,y) and R = f(x,y) should be in “number of pulses” not in milimeters.
Calibration process should involve moving router near point of my grid using Ground Control buttons ; measuring Xi and Yi and recording number of steps.
Which variable holds that value?
There are no need to calculate length of chain when we know that for given x,y we need N and M pulses for left and right motor.
Such approach maybe a significant simplification of mechanics and software and accuracy will go closer to 0.5mm rather 1.5mm .
Where in the code those values are held?
Regards 
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