It's the weekend, my gums are in the mail, and I can return to solarplate after a several week hiatus. I had said I would test step wedgies side by side and finally I did it:
The first test seemed to be a bust but then I got at least one bit of info for my $10 cost. It was: Aquatint to Positive: 3:1 to 1:1: I have 6 step wedges printed side by side on one piece of Pictorico, so I exposed each wedge to the aquatint screen from 2 mn down to 30 sec in 15 sec increments. Then I exposed the whole plate to the positive for 45 seconds. The only thing, which was a good thing, I got out of that plate was that when the aquatint screen exposure is much greater than the positive, the sharpness of the print decreases. Between the steps on the step wedge there are no clear lines of demarcation where the aquatint is close to 3:1. Steps are best defined when the aquatint ratio is 2:1 to 1:1. This makes logical sense, because you're just exposing deeper dots and shallower image. This may have application where you want the image to be softly detailed, though.
Aquatint to Positive: 2:1 to 1:3: From that I chose a 1 min aquatint screen and exposed the positive on top from 1/2 minute to 3 minutes to see what the reverse would do. This was where I saw the tonal range change. The less the exposure of the positive (1min aquatint to 30 sec positive) the more the blackest black moved up the scale and the deeper the midtones. The greater the positive exposure (up to 1 aquatint/3 min positive) I still got a max black but the midtones got quite a bit lighter as did the highlights of course. So I can see how one could control the tonal range by the ratio of aquatint to positive exposure and no curve if one didn't know a thing about diginegs (e.g. Dan Welden by his own admittance)--nothing new here because Diane Longely said as much in her book (to adjust the exposure of the positive on top of a 1 min aquatint to make the tonal range of the plate match the tonal range of the print).
With all that said, acceptable ranges of exposure appeared to be from 1mn aquatint/1/2 minute positive to 1 mn aquatint/2 minute positive, depending on how one would want to modify the tonal range. I find that the latitude of acceptability IS quite great in these plates, even if the choices within that latitude change the tonal range of the image. My guess is that one could establish a ratio and expose, say, other times as well and get the same effect--I think Camden found a time of 4 min aquatint to 2 min positive worked for him which seemed way high to me, but perhaps it illustrates the principles of ratio being more crucial than total exposure time.
This is all with KM73s and UVBL and therefore may apply to...no one...but at least I kept my promise!
PS Rajul THANKS for reporting about your kalli tests!!! I have filed the notes.