I don’t remember if I did it in FreeCad or fusion.
Basically, draw an almost horizontal line segment the length between your belts.
Then draw another line segment, and constrain it to be parallel. This is an arbitrary length. Say 300mm. Join this to the base line with 2 lines constrained to vertical of 300mm. Dimension the 2 points diagonally opposite (should be near, but not exactly sqrt(2)×300 mm) change those dimensions to be your measured diagonals. You can change the measurements of the line segments to match an actual part, of course.
Now take dimensions of the first line segment to the ordinal axis. The difference between the 2 is the amount to adjust your Y end stop positions by.
I did this with a larger square, but in hindsight, using something smaller that I could measure more precisely with calipers might have been better, since I had to guess at partial milimeter values. Of course my sketch is pretty easy to manipulate, so I could probably manage something my 180mm calipers can swallow. Significant digits is kind of what matters here, so something over 100mm with 2 decimal precision is better than something under 1000mm with no precision decimals. I know I have the sketch somewhere…
Ah. There it is.