I bumped into a topic on this forum recently where the builder used a guitar tuning app on his phone to set the belt tension. By assuring all the belts had the same (in his case D#) note, he had a method to assure that all the belts have the same tension. This seem out of the box for me, but then I found the following from this article:
Similar to a guitar string, a synchronous belt will vibrate at its resonant frequency when it is “plucked,” and the frequency of vibration is directly related to the tension of the belt. Sonic (also referred to acoustic) tension meters sense and record the belt’s vibration when it is plucked, or tapped. The tension force is then calculated based on this vibration frequency, the length of the vibrating belt span, and the mass of the belt.
So vibration seems to be one way mechanics set tension on timing belts. So if we had some data relating the “guitar note” to the belt tension on the MPCNC, maybe we would have a method of setting tension besides just guessing.