If you're looking to keep as much brightness as possible, using ColorPlace flat Light-Base instead of Behr/GliddenUPW provides a noticeable increase in ftL due to slightly less masking of the Pearl/Silver elements in either mix.
In the picture comparisons the 2RP+1RSS+1upw+1poly+2W mix suggested by Mississippi is next to the ProGrey sample while the 1RSS+1upw+1W mix I suggested is next to the ProWhite..the pictures' names are labeled in order.
Both the complex and the simple mix using the UPW is dimmer than the 1.0-1.1gain ProWhite..I'd estimate Mississippi's suggested mix to land around 0.9-1.0gain.
The simple mix as I originally recommended (using ColorPlace LightBase instead of UPW) lands a little above 1.1gain, closer to 1.2-1.3gain.
I don't suggest painting two different screen sides unless you really want to experiment. I believe the mixes (as long as you use the same flat/matte white for both) will turn out so close that a feathered line would be invisible and it'd just look like a normal screen, BUT the main point of my suggested mix is to give you equal or better performance while costing you less..so I agree it'd be better to use one or the other instead of both unless you really really like to experiment with paint for fun and experience.
Even then, Mississippi's suggested mix already includes both ingredients you could make the simpler mix with AND you could easily make his recommended mix and still have plenty of RustoleumSS and matte-white leftover to paint a small test-panel with. If you really feel an itch to see them both for yourself, that'd be more efficient than buying the RustoleumSS and matte-white twice.
Not sure I'd recommend that either though since you'd just end up seeing how much the same they are after spending around $40-$65 for the Pearl and poly that you likely can't return.
I'm making these suggestions to help, so I'm not going to be grumpy at you newB24 for choosing something different. It doesn't hurt me and you'll still have a good screen either way.
The 1quart Rustoleum sterlingsilver and 1quart ColorPlace flat Light-Base mix (with about 30oz water for thin dusters) will give a brighter image and cost less than the more complicated mix, but the higher cost and lower brightness of the complex mix aren't enough to seriously hurt your image nor your wallet.
You're in good shape either way.
I'm just grouchy at Mississippi for stepping in and talking negatively about a mix that he's never seen nor tried simply because he doesn't understand how/why it looks the same (or brighter if ColorPlace is used).
You, newB24 got caught in the middle. Its not your fault and I certainly can understand the confusion when getting different suggestions tossed your direction.
Note how much confusion Mississippi and PBmaxx are having over where each sample begins and ends..that's because there's not a lot of difference between any of these. We're somewhat splitting hairs here, and you've got a good room and bright projector with a perfectly reasonable screen-size.