Whats the recommended height above the ridge?

By far, the easiest way I’ve found it to use one of these gable mounts:

http://www.amazon.com/Winegard-SW-0012-Gable-Mount-Antenna/dp/B001DFTGUQ?ie=UTF8&keywords=gable%20mount&qid=1459190570&ref_=sr_1_2&sr=8-2