Quote Originally Posted by clough42 View Post
Just realized I never came back with an update. I eventually got the bed shimmed to within .06mm of flat and it's good enough if I split the difference on the Z height.

After talking with a mechanical engineer at work, it seems that this is typical. Flat glass really isn't very rigid. It tends to take a bow, usually across the diagonal. Once it takes this bow, it becomes very rigid and stable, but it's very tricky to keep it entirely flat.
This is almost certainly why borosilicate glass is preferred for the application. I'd be curious to hear from someone who has used it.