I have a six inch model and also one for measuring brake drums. They have a set zero that is used with the jaws closed. I think you will find it quite accurate.
That's like saying "the hook on my tape measure is always on the end."
I'm still a fan of coins as a cheap standard. You can use their thickness and diameter to check your accuracy at the 1/1000" and 1.xxx" level. You can "buy" them for less than a couple bucks!
Bill
