Checking wall thickness process is simplified by first establishing if the bore is parallel or has been bored tapered or some other weird shape. Generally they are parallel. This can be done most easily with a sprung bore gauge, you just run the gauge up the bore from either end and look how the dial moves. With this established, you just put the barrel on the WT gauge and run it up and down the full length of the rod whilst watch the dial. Any low spots you work over more carefully and eventually you know what is the minimum point.
Like Gunman, I am doing this several times per week, sometimes dozens of time, and you get pretty good at spotting the dodgy areas on a barrel set. I think I would agree with Gunman on timing.
One thing that can make it take a lot longer is the presence of serious pitting in an otherwise thick barrel. Here you are looking for the worst pitting and the approximate MWT at its worst to give an indication of whether lapping the pits out would give you a usable (or reproof-able) wall thickness overall. Obviously with the nature of pitting it is a VERY approximate measurement but worth doing none the less: it can turn a sleever into a possible reproofer or visa versa.
