What are general rules to form this superlatives: "adjective + most"?
Solution 1:
They mean the latest point reachable when you move in a certain direction:
- rightmost -> move right until you can't move anymore
- headmost -> move ahead until you can't move anymore
and so on.
The point you've reached when you can't move anymore is the point the speaker is talking about.
So in general:
- "direction"most -> move in "direction" until you can't move anymore. You will see what I mean there