My old friend "Trial & Error" works great for me. On my C-band dish I drilled a small hole on the back of the polar mount and used weed whacker line to a plumb bob to get me back where I started. At first I duck taped a piece of paper and made pencil marks on the best spot. When I felt I got the best spot a small finishing nail hammered into my deck helps me get back to that sweet spot all the time.