Agreed, it's the worst case. I suspect that the error increases the wider you open the jaw, but I have no empirical evidence to support my suspicion. Interestingly, +/- .001 seems to be common, even with the cheaper ones. But then, I don't believe everything I read in advertising, and the cheap ones don't come with calibration certs - at least not the ones I've seen.
I don't think I mentioned it, but I bought a Vista CNC pendant. I got the P2-S. Pretty nice, and way better than the Shuttle Pro I had been using. It was a snap to set up.
Gary