Andrew
With the original hub, the belt should actually overhang the open side of the sprocket by a few millimetres. It sounds wrong but that is how it was arranged.
If you have the rear on stands with the tyre clear of the floor you can spin it backwards and forwards by hand while watching the belt. When the alignment is incorrect the belt will walk to one side of the sprocket or the other when you spin the wheel one way and the reverse when you spin it in the other direction. When it's correct the belt will stay in pretty much the same position on the sprocket.
Hope this helps.
