Research into the book New Low Cost Energy Source for the Home by Peter Clegg shows a meters to wind speed height ratio. Benefits are substantial going above 50 feet in height up to 76 feet in height above sea level. Above 76 feet the benefits are not significant. But a ridge placement is dramatically different. The whole lay of the land in how wind moves makes putting a mill at the top of a ridge far greater in performance than trying to get wind below it. In any case, there is always some wind at the 70 foot above ground level spot. This is the height you have to shoot for whether youre building a tower or going for a ridge placement.