Cutting rangefinder mirror.

I didn't think this through. I now have a small 50/50 mirror i need to cut down to size.

Now if this was a window pane or a picture frame it would be cut by now, wheel cutter dabbed in oil and i'm off.

But before i attack 1mm thick, expensive enough to not really want to buy more, glass with a tool that requires quite a bit of pressure.

Has anyone got any tips?

Experiment on test pieces of course, but I am using a carbide tip scribe for scoring for mirrors. 1.3mm thick. Taskes a medium light' pressure (ok, I know, what the heck is that?). AND I can run the tip along a straight edge directly, no messing with cutting wheel offset and such (but I could be doing that wrong). I dip the tip in oil before pulling across.

This is the one I am using. Often available in the General Tool area of an Ace Hardware or such place.