By the way, Silver Ticket's website claims their neutral white screen has a gain of 1.1
Furthermore they claim their grey product has a gain of 1.0, which I believe was repeated in the post above.
I have the grey screen myself.
It's not very grey, but when I put an ordinary sheet of white paper on the screen it's not hard to see how much darker it is. Meanwhile the surface appears to be be perfectly "Lambertian" , with no preferential direction of scattering. Light leaving the surface is perfectly diffuse, just like the sheet of paper.
Based on my understanding of the very nature of the word "gain" , I doubt either of the manufacturers claimed figures above could possibly be accurate.
But don't take my word for it, the Wirecutter people measured the hell out of it and, although they still chose this screen as their #1
pick for the best all around screen, they were careful to point out the white material fell well short of it's claimed "1.1" gain, measuring more like 0.95 by their calculations.
(link still points to an article about silver ticket as of late 2019)
Given that Silver Tickets own website, and even a simple test like the sheet of paper test described above, shows that the grey material must have significantly less gain than white, there's simply no way I would believe their claim of perfect "unity" gain for an obviously grey, obviously non-retro-reflective surface.
Combining the Wirecutters findings about white (0.95) with Silver Tickets publicly acknowledged difference between white and grey (-0.1), it might be more realistic to use a value of 0.85 in your calculations. Or at the very least into your expectations.