The Main axis angle has a range of movement of approx 90 deg. The antenna swings around this axis. The Dish offset angle is the amount by which the beam it tipped downwards slightly. This angle is only a few degrees. There is a tendency for dishes to sag somewhat and elevation angle scales may be inaccurate by 1 or 2 deg.
Some minor adjustments are therefore necessary once satellites are being observed.
The Dish offset angle must be set exactly. A plumb bob, i.e. fishing line and weight plus measuring rule and calculator or tan tables is an improvised alternative which can be quite accurate. Check the mount tube is exactly vertical. Apply the inclinometer in several places around the tube. All should read 90.0 deg.