Good point, I didn't read carefully enough to notice speaking width not diagonal. A difficulty, since screens are commonly given in diagonal inches; using width is atypical.
D = 1.147*W
70" W ==> 80"
75" W ==> 86"
I still think that size range is the worst of all choices for projector...
That is the worst of all worlds: TV size with Projector quality, cost, complexity.
Buy a 4K OLED 75" direct view for the same cost of a projector and screen and have much, much better quality viewing.
Everything: TV, Movies, videogames. On average, about 2 hrs a day with the projector watching TV.
Use low lamp for most viewing to preserve life, and bright lamp for stuff I care most about.