This is on the MF12 I believe? I think you need to have it down in a little, the belt should contact just the flats of the pulley, not where it starts to curve. The more surface area on the pulley faces, the better.
I think you need to go through the whole Vari-drive adjustment procedure.
The rear Idler pulley needs to be flush to the top of the frame to 1/4" below when in park, if not add or remove shims till it's right.
You will probably have to add shims with a new belt installed.
I don't there is any set spot for the rear belt to be in because as the belt wears it changes.