Ok, the lightbulb finally turned on.

I think the most ideal way to meter UV-exposures would be in watt-seconds, or joules (just like the holography folks).

So I can say, my carbon tissues needs a total of X watt seconds of energy to be properly exposed. This value then could easily be translated to black-lights, cloudy days, sunny days, etc.; as long as you are keeping track of the UV levels throughout the exposure & keeping a tally of the total energy.

Does that seem reasonable? Any pitfalls I'm missing?

I've been playing around with the meter a bit. Today at noon, with the bright sun veiled in thin clouds I was getting readings as high as 2.0 mW/cm². In the morning, thru my living room windows, about 0.5 mW/cm². In the evening yesterday, when the sun was just below the horizon, I was only able to eek out a reading of 0.1 mW/cm² from the sky.