Depends aswell on which latitude you're on. Don't forget the coriolis effect that turns everything to the right on the northern hemisphere and opposite on the southern. this makes the geostrophic wind blow parallel to the isobars with the high pressure to the right. And the greater the pressure diff on a given distance, the stronger the wind. I don't know how to calculate this though. I could be wrong on this though, so you better check it up in some book.