How tight is too tight?

I just cleaned my 12" GSO mirror and I'm not sure how snug the mirror clamps should be. Better to get it right while it's out of the scope. It's a full thickness mirror and the clamps have about 6mm of rubber cushion and I don't want to pinch it.

You should be able to rotate the mirror in the cell without binding. The clips should be above the mirror about a playing card's thickness.