How accurate is this for measuring camber???

FTC

Just a dumb welder
Dec 18, 2012
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Hawaii
So I figure the rotor has to be pretty damn close to parallel of the hub, so I've been just throwing the angle finder on my rotor face to get a reading. How close to actual camber does this read? I eventually will make a bar to clamp to the tires but for now I'm just using this. If not for anything else at least I know my camber curve which I am very happy with. Any input is appreciated.
 
Forgot to attatch pic
yrybezy7.jpg