Camber / Gauge location

[RBuhr69]

I have one of the Longacre Camber gauges from my racing days but it has the magnet end version that would normally stick to a steel hub....Well I have Wilwood aluminum hubs so the magnet will not work obviously......

So I was searching and saw someone placing the gauge actually on the Rotor. So is this an acceptable location to place it on through the wheel to get a reading or would that be an inaccurate reading versus in/on the actual spindle end itself?

Thoughts?