You can measure camber just as accurately with a spirit level IMHO and yes, you need to do it on a level floor or zero the gauge against the surface you are on.
Cheap spirit level with a couple of holes tapped into it and 2 bolts does the job just as well.
Kevin
This guage is calibrated in one degree increment and decrement.
How do you accurately calibrate with a bubble between two lines and no calibration markings ?
With about the same accuracy, as I said...
Just because it will display to n decimal places it doesn't mean that's the accuracy you are achieving, especially when considering the effects of not having dead level ground, which you can do nothing about with either solution. 1 degree shifts the bubble in a spirit level quite a way too.
If you need to get it spot on, you need to go to a decent depot. Something that you can adjust on a DIY basis will reduce the amount of tread you use getting there after a wishbone change, or allow you to experiment with the settings once you have a feel for how your garage floor affects the readings.
Kevin