My personal experience in etching glass with HF is limited to etching of
Pyrex in 6:1 BOE. In this case, the etch rate was ~2 microns/hour.  At
this rate you probably don't need to worry about over etching, and
should get acceptable results on the first try if you can tolerate the
undercut.  If you want to etch faster, use straight HF.

Roger Shile