I have the same problem guessing what color is going to convert to a good Zone V gray. Depending on the scene, I usually use the meter in incident mode with color, and then check highlights and shadows with the spot mode like you did. If the SBR is out of range for fashion/commercial-style work, I'll try to augment the lighting to bring the ratios under control. Obviously, that's more difficult with scenics.
You can tweek E-6 contrast a bit (not like B&W, of course), but with rollfilm, that's probably not practical. I think I'd specify normal processing with what you have, as your spot metering sounds reasonable.