I would never use the body lines only. Especially with cut fender flares! You never know how consistent they are. And that does not isolate suspension from body mount, frame straightness, or tire consistency.
The most (really the only that I know of) conclusive point to measure for suspension height is the gap between the top of the axle tubes and the bottom of the frame rails.
This way you can get a suspension-only corner by corner number.
Any variation between the four corners is your lean.
The hardest thing though, is to determine whether a lean is coming from the front or rear. It might look like the rear and be a problem with the front, and vice versa.
Of course, you need to eyeball things like bumper gaps too, to make sure you are not dealing with things that are crooked above the suspension, but that's why we like to use the gaps between the frame and axles.
Paul