Pretty obscure and unlikely to be of interest to many folks, but it turned out to be extremely helpful so I thought I'd post it.  This is specific to the Benchcrafted Tail Vise.  With the rails in place this jig is placed inside the rails and slid up against the end cap.  The center stud extends through the thru-hole.  You can then hold the jig in place and slip the flange over the stud to place it exactly on centerline between the flange and the nut.  Ideally you'd do this before drilling the flange bolt holes, but since these bolts are secured on both ends they can have some play.  So if your pre-drilled flange nut holes are off they can be widened using a larger drill size or a rat-tail file to get the flange centered.