I have seen them tall and short. I make my own and make them real tall to get over the barrel block. Also I don't like my cheek to touch the stock. I believe around 6 degrees of angle gives you 20 minutes. I have some from Bat that are about an inch tall and some that are only about 1/2" tall.
First we need to know the spacing between the rings. In this example we will call it 6 inches. To find the correct amount to raise the rear ring we divide the 20 inches of elevation we want back by the number of inches in 100 yards (3600). This is then multiplied by the ring spacing in inches (6). This looks like (20/3600) x 6 = .033". The rear ring should be raised .033" for the correct adjustment. As mentioned, this same principle applies to Unertl mount systems if the maximum amount of `up' adjustment is desired. The Unertl scope in the pictures is mounted on a base which has the front mounting area surface ground .075" lower than the rear.