Honestly, I've never worried about enough about erosion to measure it... I simply look down the barrel and if the first couple inches isn't burned flat (or frosty from fire-cracking) and she shoots I'm good. I've fried out a few pipes and none of them gave a hoot about the little measurements until the barrel was about fried out anyway.
You will see movement in the throat, but the wear is not only at the very origin of the rifling so measuring it is really a false comfort. You really need to be able to measure rifling depth for the first few inches to make an educated guess as to what you have left for bore life, so break out the cerrosafe or just eyeball it like I do and let the groups tell you what's up.