Scale rule marker

I can use various CAD programs, but since I don't use them often (too slow), I sometimes use a scale rule and a piece of paper.
I find it gives a better feel for things.
But I did make one goof while using one of those 3 sided scale rules: I somehow used a different scale on some part of the drawing.
Every time I pick up the scale rule, I have to turn it over, looking for the right scale (It's always the last one).
I propose a little plastic 3 pronged oojamadoobreything that allows the user to point a red marker to say "THIS ONE, STUPID".