I've been wrestling with this a bit too.
The idea of working back from the final print is an important concept.
I added an extra piece of info to my equation, the viewing distance.
Format:Focal Length = Print Size:Viewing Distance
The Idea is to keep the perspective of both "the final view" and "the cameras view" at very close to the same ratio.
I came to the conclusion that one of the reason that 85mm and 105mm lenses are so popular on 35mm film for portraits is that this was driven by the "normal" print sizes of 8x10 and 5x7 being viewed "in hand" or "on desk" or "in a hallway".
This thought is pushing me to use fairly "normal" focal lengths (35 to 50) with my 35mm film because I'm working toward creating more artsy stuff for larger spaces like living rooms.