I would put the jig on one side of the ply first Mike and set the gap. Then you have to decide which side you want to register from, 4 1/4 from the bottom or 5" at the top. If you want uniformity from unit to unit I would add two arms to it, one at each end with a cleat to hook the bottom of the gables. I would use a spacer block to set the cleats at the right distance and that should give you accuracy within a few thou.
Someone I consider a master woodworker once told me that a master woodworker is not someone who never makes mistakes. He is someone who is able to cover them up so that no one can tell.