I think it differs for everyone, but I can only get a good seal by having the top of the mask (with the bendy metal bit) well down my nose, so the metal bends d

I think it differs for everyone, but I can only get a good seal by having the top of the mask (with the bendy metal bit) well down my nose, so the metal bends down into the "crease" bit on the outside of my nostrils. If I have it higher near the bridge, inevitably air comes up my cheeks and out to fog my glasses. My nose is fairly big, I suppose.