Thanks YiannisWell, let's do some math...
80'' screen in 1,78:1 means 1,80 s.m.
Grimmys Grey's gain is about 0.25 (paint plus finish)
Icon Grey's gain is about 0.33 (paint plus finish)
Granite Gray's gain is about 0.35 (paint plus finish)
Benq on low lamp and cinema mode (without calibration) outputs around 850 lumens. So:
On GG you 'll have 10,97 FL, on IG 14,48 FL and on GG 15,36 FL.
Considering over time that the lamp will lose power, i would say Granite Gray or even a tad lighter would be the wiser choice.
I went ahead and painted the screen Granite Grey and the result is awesome. Much deeper colors now... I should have done this earlier