It comes down to personal preference.
12ft from 120" diagonal 16:9 is 39.9 degrees
That is the THX recommendation of 40 degrees for 16:9.
With a cinemascope image it is just a bit more than the 36 degree recommended minimum for the back row in a cinema.
So should be fine.
THX recommend 40 degrees for 16:9, 52 degrees for cinemascope using constant image height. For the back row in a cinemascope cinema THX minimum is 36 degrees recommended 26 degree minimum.
Typical commercial cinema average 54 degrees with a cinemascope image.
20th Century Fox when developing cinemascope in 1953 came up with 45 degrees as optimal best seat due to the limitations of film at the time (film grain, film unsteadiness jump and weave, soft focus). With original cinemascope 2.55:1 that would be 3.08 screen heights which is almost the same as the SMPTE reference of 3 screen heights. As film projection has improved people can now sit closer without the limitations becoming obvious.
Personally I am about 9ft10in away from a 125.375 inch diagonal 16:9 screen. So the screen occupies about 49.7 degrees of my field of view. Seating distance to screen is however very dependent on personal preference.