I'm not sure a single scale is really going to "fit" the many shades you have. On the other hand, trying to describe all the different varieties makes it complicated, and complexity makes it less useful.
One important distinction in addition to the "level of RP" is the "causality of RP". A lot of in-game decisions will often have both IC and OOC motivations. For example, from a selection of ICly sensible areas to move to, one is picked primarily by OOC reasons ("more targets to shoot = more fun" or somesuch), and then an IC reason is found to explain the decision. Othertimes, decisions are motivated ICly and the "OOC fun" is derived from the IC decisions with IC reasons and IC consequences.
A recent example was the Arek'Jalaan thing (I got the spelling wrong again for sure). While some characters will have no issue at all with it, others might. Is it "normal" in such situations to go "meh, I want to have RP, so I'll adjust my character a bit so I can get in there" - which still is fully IC, fully believable, fully RP, but motivated by OOC reasons - or is it "normal" to go "I might get some RP out of it if I change my character, but I also get RP out of it if I don't, so I'll just RP the wayI feel is right without worrying about the OOC stuff"?
For most entities, decisions have both motivations. The question is what is more dominating: Are most decisions done primarily for OOC reasons, and then IC reasons to explain the decisions are found? Or are the decisions based on IC considerations primarily, and OOC considerations usually take a backseat?
Neither of the two is (er, the various places on the continuum are...) "wrong RP", it's just different kinds of RP and different kinds of enjoying the game.
(It's incredibly difficult to discuss "RP styles" without at least sounding to someone like you're saying "urdoinitwrong", sadly. :-/)