That's interesting about paragon, the one 44mm head tube I looked up said 43.5. Maybe they went a little bigger on some parts, because 1/2 mm is a lot of material to remove.

I don't think Reynolds aiming for the exact dimension is a good idea. I wouldn't have thought anyone would do that. Certainly Reynolds BITD was undersize. Maybe their production customers wanted to do less reaming.

Facing the columbus head tube is a good idea. It looks good, but can't hurt to be sure. I'm pretty sure the Paragon head tube I have is square, I might mess it up more than anything if I tried to true it.