The math for your 1080p projector setup doesn’t sound as bad as your guess for the 4K screen. A 135" (342.9 cm) 1080p screen has a pixel density of 16.32 PPI (6.425 PPcm). That gives a pixel threshold of 211" (535.94 cm), and going with your 10’ (304.8 cm) figure that would put you at about 57% of the average eye’s resolution. Though now I see you’re saying that it is only the distance at which it doesn’t bother you with a natural image, but subtitles or other things you can still make pixels out. So that may not be very accurate either. I was going to say that if that 10’ (304.8 cm) distance was the threshold for you then you wouldn’t even be bothered by 720p at that screen size, because ~57% of 16.32 PPI (6.425 PPcm) is around 9.28 PPI (3.654 PPcm) and that would work out to a threshold resolution for you of only 1092x614 for a 135" (342.9 cm) screen at 10’ (304.8 cm)! But since you clarified what you can see at 10’ (304.8 cm) we can ignore that now. 😉