Interesting comments so far. Thanks for the ideas.
I think the first thing I need to say is that I was seriously taken aback by Crinklaw's findings that after five years an unwashed mirror needs as much more exposure as a freshly washed one. (Reflectivity was off some 45%) Not only did he cite some studies, but he also had the proof of the pudding---his program worked when he factored it in, but did not work without it.
My mirrors look good. I mean, they have never been touched. No fingerprints, nothing, Since dew is not generally a problem, and when not in use they are covered with two plastic bags (not perfectly tightly--air can probably get through the overlaps, etc.), and a cotton cloth wrapped around the tube, they look generally clean. I must admit, they do not have that "new telescope" look. And a five minute exposure now gets me, I think, the same ADU value in the background sky as I got two years ago when I first installed the camera (in general--I mean the nights differ some but in general, I get the same reading). But, I kinda believe Crinklaw has his findings on his side. That, of course, is why I want to test.
Cookjaiii, your tutorial link is interesting, but I think it supports Crinklaw's observations more than not. It seems that (in the tutorial) they are comparing mirrors that are "washed in situ" to the new mirrors. They do not seem to compare new (or newly washed mirrors) to unwashed. One of the graphs (readings for 2011-2012) shows a jump of about three or four per cent from the before to the after washing after one year. If you continue a three or four per cent loss year after year, you lose 20 to 30% from year one to the end of year five. And that is for a mountaintop in Hawaii. Not for my dusty dry desert. And they note that the conditions in Chile (dry, dustier) mean more loss than in Hawaii.
I think, I can do both Frank's and Dan's tests easily enough. (and probably good to have two or three ways to test. I don't think Tom's will work for me, since I am talking about a remote desert observatory, not a place full of reflectivity testers, and meters, and all that (which I do not know how to use anyway).
I think Dan's admonition to somehow calibrate the flat panel (to be sure it is putting out the same before and after the cleaning) is important, too. How could I go about that?
And, of course, Frank's idea depends on me having the same sky before and after.... How do I go about that? Or how do I expose to correct for that?
Keep the ideas coming.