In this drawing the belt is too short for the particular combination of axle to axle length and the diameter of the end wheel. This is called wrap and provides a lot of friction/drag. Using your drawing if the belt comes off Point A on the top and separates from the wheel on the second Point A (middle of the wheel) that will be just about a 90 degree turn. That is very little friction and would be perfect except it then has to make another 90 degree turn to get back on top of the other wheel to get back to the running surface. That second 90 degree turn will be a lot drag and the mill will not be as smooth as it should. On the back end there will be some hop as it comes to the top of the mill. Almost like a ramp.

Ideally, counting from the top of your drawing the belt should leave the wheel around the 7th arrow. That will be the using slightly less than half the wheel on both ends. That is ball park.

If there is a mill you have access too measure from axel to axle, check the diameter of the end wheels, measure the belt length, and/or count the slats taking into account the spacing. Then build from there. With the really nice mills all these measurements are really close. The frames, the slat materials, the belt material and the bed wheels bring uniqueness to the mill from the individual builder. EWO