I have never trusted those riveted on end caps, now I'm glad I didn't. I always measure from a mark further down the tape.
The tab is for inside/outside measuring. If moves (the thickness of the tab) for a reason. I always cringe when I see people let a tape slam back into itself when winding it up.
You have to trust your tire guy's gauge. Consistency is more important to me in this instance. As long as I like the tire feel and wear I can create a repeatable condition with that one gauge.
I just skipped from the wank thread to this and made the obvious incorrect mental picture. Send me some bitcoins for that evil.