How have you marked your ground glass?

[Keith Fleming]

And in the center of the ground glass I put a small "X" in pencil which serves for ensuring my loupe is properly focused on the ground glass itself.

Keith

[Ron (Netherlands)]

I always want my ground glasses clean as possible...i.e. with no markings at all

Btw you can easily clean a dirty ground glass by using kitchen abrassive like Vim on the frosted side. VIM comes in powder or fluid.
Reparing an old groundglass with spots or streaks on te frosted surface can be done by repolishing with micro grit powder (aka silicon carbide) > https://www.flickr.com/photos/zorki_...57660810886723

[Bipin]

I use Letraset to make my markings. It looks "professional", and you can easily make precise dotted lines with it, or make alphanumeric markings. Right now, I have a + in the center, with two solid horizontal lines for 6x12 and a dotted line just above the lower solid line, to mark a 2.33:1 image area, which is a favourite aspect ratio of mine.