Actually, I would add at least 2 inches to your head measurement. After fiberglass and padding in the helmet, you'll lose that inch. You want some overhang, so your chin isn't poking out the bottom and you can talk without your mouth hanging out. I find 290 mm tall to be a good size. And remember, if you're just wearing the helmet, it'll look too big. When you add the armor it'll look proportional. Good luck!