From my opinion base, I put both tubes back in the tacking jig, line them up and lightly snug the clamps. Then I turn off the shop lights and stick a small LED MagLite down each tube, one at a time. If I don't see any light sneaking out at the intersection, then its is spot on perfect. If there is much light is sneaking out, it is time to do a bit of filing to correct the fit or to cut a replacement tube after sorting out where and how the error occurred... and just relegate the miss-cut one into the barrel for when doing a slightly smaller frame.