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i'm thinking of ohp markers, though difficult to remove, creates a very thin line...
Not as "very thin" as you might think. This is how "fine" a 0.6mm OHP marker line will appear in the viewfinder when drawn on a 24x36mm (FX/35mm) focusing screen ...
On a DX format focusing screen they will appear 1.5X thicker than above.
There's a reason permanent grid marks on 35mm SLR focusing screens were "etched", probably only about 0.1~0.2mm wide.