Video Camera ISO rating

Howdy again,

I have another question for the cinematography experts. I'm trying to apply what I'm learning about lighting for film to my video production efforts as well. And a lot of what I'm learning involves the use of a light meter to determine my lighting ratios and to help me decide on my exposure.

In another class I had, we did a test to measure the equivalent ISO of our video cameras. We'd shoot a 18% grey card, and then light it until we got A 55 on the waveform monitor. We would then take a reading, and dial the ISO on the meter until the stop on the meter matched the stop on the lens.

The problem with this is that our "measured" ISO ratings for our video cameras fluctuated wildly, even with the same camera from day to day. And thinking about it, it seems that changing things like pedestal and gamma, etc, would throw this test off, since we are using the middle gray point to calibrate it, and if that point moves away from the middle of the exposure range, it would yield inaccurate results, right?

So what can I do to figure out the ISO of a given camera? What do you guys do to ensure that you're able to get useful readings out of your light meter?