As other have said, you don't focus on the mirror. You focus on the object you are looking at, 30 feet away.

That said, I do not like progressive lenses for helmet/eyeglass mirrors because those lenses (at least the pair I tried) focus clearly at the center of the lens but poorly at the edges. I now use bifocals which are sharp to the edges and give me an excellent view behind with a helmet mirror.

Ben