Just thought of something: that bubble camber gauge I use has a thumbscrew to move the bubble. The mount for the bubble level is spring loaded. This means the tip of the thumbscrew can dig a hole in the aluminum if you aren't careful and this can change the zero point. Since each full turn of the fine thread screw is one degree, 20 or 30 thousandths of wear can make a pretty big difference.
I generally set the gauge against the wheel, check my bubble position, then press the pivot to lift the screw off the aluminum, turn it two or three segments whichever way I need to go then let it back down and read the bubble again.
