You cannot use the 1080st on a white screen of at/under 85" without suffering from Xerox-ed Eyeballs.
At 85" and at the furthest throw available for that size, you'd still be getting a huge 41 foot lambert of reflected brightness! 15 fls is standard for a Theater setting, and 19-21 fls for very moderate ambient light.
You'd be getting "Well Lit Classroom Presentation PJ" brightness, and that Sir would also grossly affect your Black rendition, and send light bouncing off every room surface like they were back-lit.
You need to mix the SW paint with a Neutral Gray of approx .80 gain or less, and run the BenQ on Low lamp. Even then, you'd be well advised to go to some extra lengths to offset the amount of light that will come off the screen by painting the Ceiling and Side walls the same shade of Gray as the screen.....minimally. Better still....2x-3x- darker...in a Interior Enamel Flat.
Lastly, please be aware that the 1080 has no Lens shift, and only a tine amount of Image Offset, so unless you want to be required to place the PJ at the same height as the Top edge of the screen, or use Keystone Correction because you were forced to tilt the PJ several degrees, I'd be the first to suggest you instead jump up to 100" diagonal (24 fls @ 6' throw and .80 gain)
Bigger is better....especially if your a Gamer. Otherwise, chose a similar PJ with Vertical Lens Shift (BenQ 1070) and place it further back.(10') That will actually be good because it will remove the PJ from being in front of your viewing position.
Whatever screen size you do choose, have the boys at Laird cut it to size for your selected Diagonal size (98" diagonal = 48" x 85") or go smaller (90" diagonal ) and have the sheet cut 48" x 82" and use the extra 2" on all sides to allow you to screw the screen directly to the wall.. Apply Trim over the 2" area to Cover the Screws and your all set.