Manual calibration space between motors

Is there a reason that i couldn’t just measure the length of my top beam and then use a caliper to measure from the end of the beam to each motor and then just subtract that amount from the total length of the beam to get the spacing between motors value?
My motors go about 1/2 inch into the space between the joists in my ceiling, so the chain stretch method wont work - there’s no straight line between the tops of the motor cogs.

Measuring the length between the motors is tricky with a tape measure and it’s also not very accurate, but measuring the full beam and then subtracting the edges seems like it should work.

Also - am I supposed to measure from the center of the cog, or the inside / outside of it?
I’m assuming from center of gear like this:


That seems like a good way. Others have measured from outside of gear box to the outside of the other gear box on their motors then subtracted 40.4mm. you can try the two and see how they compare.

This might be elementary, but if you are supposed to measure center to center, when subtracting from your full beam, measure to the right of each shaft for accuracy.

You could temporarily detach your top beam for the measurement, that way you can do the chain stretch.