Hi Jushua,
The compass is a straight wood beam stick, and It seats into a sprocket notch. Is imune to chain stretch, chain tolerance, and tape measures. So it seemed to me that using this to mark the board is very repeatable and might shed a new light on the calibration question.
On the other hand, this drawing construction reveals the line crossing the top beam 90 degrees across the center point. That is a simple task, and a significant result.
I sought this alternate technique to step back and review my MaslowCNC from a different angle. The analysis of sources of errors reveals that this center line is a key to tune chain tolerances. So I wanted to find it without the chains. 
But it is possible that a very small lateral error on this trace may limit chain tolerance correction below necessary tuning accuracy. So I’m currently studying the sensitivity of the drawing error and chain tolerance.