First... dish must be rotated to its highest point. Various dishes are built differently and the best point to set the inclinometer will vary. Wherever you set the inclinometer,be sure it lies flat... no rivets or bent spots, etc.
Elevation: Inclinometer goes on the big pivoting bar that attaches near the pole. OR... some dishes have a back plate that is at 90 degrees off-set.
Declination: Inclinometer goes on the back of the mounting ring. Or place a board across the front of the dish and measure from there. (Think of the declination as an additional off-set from the main pivot bar. Declination moves the dish, not the main pivot point.
Maybe this drawing will help:
Drawing credit goes to repentnow(dot) com