Note...
A "prime focus" dish is pointing at where it looks like it is pointing. This is a dish with the LNB mounted in the center of the dish.
An "offset" dish is pointing higher than where it looks like it is pointing. Offset dishes can be offset 15 degrees, or 18 degrees, or 20 degrees. It depends on the specific dish. They of course do not include this information along with many dishes and I don't know how they expect us to aim them without knowing this!
So use a dish pointing calculator, find out from the dish manufacturer what the offset for your dish is and enter that into the calculator, then use an "inclinometer" to adjust the elevation on the dish. If there is a bar on the back of the dish which is parallel with the dish, you can use that. Otherwise put a board or whatever across the front of the dish to get the angle. Some dishes have no "parallel to dish" surface or bar on the back side...
Dish pointing calculator which includes dish offset in calculations...
Javascript for antenna lookangles calculations