What do you guys target for horizontal and vertical parallelism accuracy when mounting (square) linear rails?

I've had mixed success trying to mount my Y and Z rails using digital calipers and a dial gauge to test how parallel they are but I think the quality is a limiting factor. In theory, my digital calipers are accurate to 0.01mm but I have trouble achieving the same measurement twice for a fixed distance. In reality, I only trust their accuracy to 0.1mm. I place even less faith in my dial gauge / magnetic base stand set (which seems to just produce random readings as I slide the bearing block back and forth).

What would you guys consider to be acceptable mounting parallel accuracy for rails over a 30" or 48" slide?

The rails seem to function ok if they are parallel to 0.1mm in that the stage slides up and down without that horrible grinding noise that seems to occur when I get it too far out of tolerance. That doesn't mean it will perform well on a finished CNC machine though.

Do you add any adjustment screws for fine tuning? I find it very difficult to make 0.05mm adjustments to rails by hand. I am wondering if there are some other tools to help me....

I achieved the best results (so far) by just using machinist parallels between the rails as I drill the screw holes in the mounting surface. Still not much better than 0.1mm though.