Johnielvis, now I'm really confused! But, it sounds like the boat's still holding water, right? Can you extrapolate on this a bit, it sounds interesting but I'm not sure I've wrapped my noggin around it yet.

Chan, the easiest way to do this would be to get a gray card and measure that with a reflective meter outside of the camera. However, an incident reading in the same position as the card should give the same reading. Then, measure the image of that gray card being projected at the film plane of your camera with an incident meter, or if you're really small (or have an enormous camera) you could take a reflective reading.