I measured a paragon (43.74mm) and one of the tapered ones (43.76mm) with telescoping bore thingies and a verified micrometer. My puck is 43.42mm measured with the micrometer. I face both ends of the tube on the lathe and hope the top cone centers it enough to make up the few thousandths that it's undersized around the periphery. I tack on the centerline so I don't think the gap hurts me. If I was to make a new lower puck, I'd make it slightly bigger than my current. I would also add some way to align the top and bottom holders for the headtube. As mine is now, I have to do a bit of re-alignment if I move the cones/pucks to soak the flux off of them.

Drew Guldalian's new fixture uses an expanding mandrel for the lower puck which seems pretty smart.

Post up some pics!