I think I need to describe how I'm using the jig to align the frame. It starts at the rear axle which is held by spacer blocks that hold it at 2" centered above the jig surface perpendicular to the centerline and level. I then use two spacer blocks to hold the maintube at 2.707" above the jig with side wings that hold it centered. I then just clamp the sidewings to clamp the tube.
The next step is to align the maintube so the headtube is perfectly vertical. To do this I use a piece of straight 5/8" rod. I then made a set of spacers for it by drilling a hole through a piece of wood and mounting it on the rod and turning it in the metal lathe. I made two spacers that press fit into the headtube out of oak to hold the rod. And a third out of pine that is exactly 2" in diameter as a reference at the top of the tube.
I locate the tube front to back by locating the front of the 5/8" rod at a mark I made on the jig. I rotate the tube to align it vertically by using a square and sliding it against the tube and then sweeping it back and forth against the spacer at the top of the rod so it scrapes evenly on both sides. I now know that I'm aligned to within a couple thou.