I personaly use range rods to dial in a barrel and I understand that their will be a little slack that won't be read , I've never had a problem with it though.
I have also seen a gunsmith chamber is barrels about the same way Chad does only he just indicates on where the bullet enters tha barrel (throat)and in the web area and reads off the flats. His theory is that it doesen't matter if the barrel makes a 45 degree run , if the bullet doesen't get strated strait it'll never shoot strait , he doesn't concern himself with any barrel curve , all hes worried about is making sure that the bullet hits the same spot each time (bench rest) and that can't happebn unless it gets strarted strait.
I rechambered a savage 308 heavy barrel once to 300 WSM
,I set the barrel up and indicated on the old chamber and just foir kicks I inserted a range rod in the bore on the chamber end and was amazed how bad it was out probably .020+" , so i reindicated the barrel off the bore then bored the old chamber out to true then rechambered.