I may be seeing it wrong, but I think Peter B. is on to something. Assuming no deflection, there can be a lot of variation along that nineteen plus inches between the two support points of your bearings. I believe the gauge might work best if it was zeroed right on one of the support bearings, then make the measurement at that same point. But, then you'd have to move things to get the barrels on the jig.

Again, sorry if I'm seeing the jig incorrectly, I like to see shop solutions and how folks work through problems.