I don't think I'd use the 3mm reed for something lightly boned because it's pretty flexible, but something between 3mm and 1/2" (which is very sturdy) would probably be good. (Sorry to mix units, but that's how it was sold so it's how I think of it.) For fully boned stays, the 3mm seems to be enough. You might ask Ginger too because she used reed for her white wool stays, which I think are either 1770's or 1780's.