That's not offset angle, that is horizontal field of view. See, for example, here:
Viewing Distance Calculator
myhometheater.homestead.com
My two cents is figuring out your target field of view range is the best way of then figuring out what TV-size/distance combination to set up.
As always with such things, it really comes down to personal preference. But I think a good starting point is THX's home theater recommended distance range of 36-40 degrees. If you haven't tried watching a decent 4K TV in that range, you definitely should.
At 8 feet, 36 degrees is 71.6 inches, and 40 degrees is 80.1 inches (diagonal on a 16:9). So that would be the target size range for someone trying to be in that 36-40 THX Home Theater range.