Thanks again everyone.
After reading about a million more threads, I guess the pattern I am seeing is that the ideal level of "greyness" seems to primarily be a function of the projector's ability provide a high contrast image, while the ideal level of "gain" seems to be more a function of the ambient light levels in the room and the projector's ability to provide brightness on your chosen screen size.
EDIT -- I just noticed that Alan said the same thing 10 posts earlier, but with fewer words and more clarity. D'oh !!
It seems to me that I have an ideal situation w.r.t. projector brightness/contrast vs. screen size, but a less than ideal situation w.r.t. ambient light.
Intuitively it seems that a greyer screen should be better at controlling ambient light, but it doesn't seem to work that way -- presumably because the greyer screens also have lower gain ?
Anyways, since I clearly (a) need a starting point, and (b) either need to cover up a lot of green or cover up some masonite or whatever (the parkland products don't seem to be available at HT but I will call them and see who else up here carries them) Randy's suggestion of starting with a pure white is seeming like a pretty good idea.
Emerging consensus on the boards seems to be that pure silverscreen is a bit darker than ideal for most rooms, but punching up the gain of the SS with some "mixings" (a white pearl plus clear poly) seems to make a pretty good finish without requiring any particular painting skill.
Past that, it seems like doing a bunch of testing seems to be the only way to get further improvement.
I still have some doubts that what I wrote at the front of this post (greyness optimizes for projector contrast, gain optimizes for room & projector brightness) is really that simple, but that's the best I've been able to figure out so far.
I am really not good at "which one do you prefer ?" tests -- I tend to pick and choose myself into a corner and end up with a strong preference for something that looks awful, even to me
EDIT -- Randy, when I went to the Dazian site and looked at their "white" projector cloth it seemed to actually be a light grey PVC. Is that what you have ?