Proper viewing distance is whatever distance the viewer wants to view from.
I've some landscape prints hanging in my local pub. People look at them from a distance, then go closer and closer, and as people have said to me, they were either trying to determine whether they were paintings or photographs, or wanted a closer look and then felt drawn-in to look ever closer at the detail. I don't regard that as a bad thing. People tend not to do that with portraits.
