Hi Darrrell,
ah, I see now! Do people do that ?
Sorry mate, I'm so into my own headspace I never realised that you might not be using image capture software.
In Photoshop you should still be able to see the mean, I guess you can see the histogram (Menus: Window/Histogram) but unfortunately Photoshop only shows the screen histogram which is only 8-bit even if you have a 16-bit image loaded. In CS2 anyway, perhaps CS3 is more friendly.
You can still get an approximation of 30% maximum, only the mean will be 30% of 256, rather than your 16-bit value. Try going for a value of 65-70 in Photoshops 8-bit histogram and see how you go.