Originally Posted by jrcrunch
i will have 2 rows of seating
what is the maximum and minimum distance for watching on a 120 inch screen with a 4k projector screen?
If your screen is 120 inch diagonal 16:9, then your screen height is 58.8 inches.
Ultimately it's down to personal preference and comfort, so experimentation is the best way to find out how close you can comfortably sit for long periods with the screen you have at the height it is mounted - the closer you sit the greater the vertical viewing angle becomes. 15 degrees is the recommended vertical viewing angle for comfort, but 35 degrees is the recommended maximum, so there is leeway there.
As for a viewing distance 'range' for 4k, Sony recommend a 1.5 x Screen Height seating distance ratio for 4K, so to get the range you could approach it in one of two ways provided that you are watching 1080 upscaled to 4k since that currently all your source material is probably 1080, and I would think that would be the limitation from a source quality perspective.
Combine Sonys recommendation with THXs recommended range of 2.4 x SH (optimal) to 3.68 x SH (back row) which is based on viewing angles/immersion, and their front row limitation appearing to hinge on good quality 1080, to give you a range of 1.5 to 3.68.
Or reduce the back row by a similar amount as the front based on Sonys recommendation to give you something like 1.5SH 2.78SH.
But the closer seating distance will rely on good quality source material.
Having said that, THX have also said that they feel that a 60 degree viewing angle (2xSH) is probably too wide for films (this is also SPMTEs closest recommended seating distance), but I'm not sure if they are basing that on a comfort perspective (too much eye movement) or an image quality perspective. Maybe a combination of the two.
Considering you're using upscaled 1080 rather than 4k as your source material, that may ultimately determine how close you want to sit, so we're back to experimenting to see what works best for you.