The pump pulled down to around -20, but that was at 6500 feet elevation. If I'm doing the math right, at that elevation about 6 in. Hg has been lost. So that would suggest it came close to the 28.3 target.

I think -20, even with altitude compensation, is a bit low. I'm at roughly 3800 feet and I only lose about 1.2-1.4 in Hg when I pull a vacuum at home with my pump. My brother lives at a lower altitude and I can get the full 28.3 at his house. The important thing is that it holds whatever vacuum that you are albe to pull.