I read this post several times and still haven't totally figured it out, but I think you are talking about using a dedicated flash unit with a camera in the through the lens metering mode.
The communication between the camera and the lens in that situation basically is the camera feeding some basic exposure data to the flash unit which then determines how long the flash duration must be to bring the exposure up to what the camera considers "proper" exposure. If you crank in adjustments in the camera, say adding an additional half stop exposure, the camera takes that into consideration in the information it sends to the flash, and the flash exposure is thus modified.
The 18% gray reference is a convenient equivalence of the overall exposure. The actual exposure obtained may vary depending on the particular camera/dedicated flash unit. It's always a good idea to run a series of exposure tests to verify if your camera/flash combo is "getting it right".
Now then if you run in manual mode on the camera and flash, you do all the exposure calculation work, but you also have complete control.
Did I get close to answering your question?
