PS. Best place for the SB 600 for flash assist? I could put it on a tripod right at he edge of the stage. Or perhaps at the front row. In the center (seems more logical than at te side, where it would, however, be out of the way.
As for setting exposure by putting the spot on a neutral gray. There's a good chance that there is no neutral gray -- either because of the stage setting, the costume, or the set lighting. What then? I probably can't put a gray card on the set
The art director's husband, who, as I said, has gray hair. I don't think that's exactly neutral though, and he is not always on the set, anyhow.