The way I've done it, although I really don't think its optimal, is to do it on a very individual basis w/ no formal guidelines. What I've found useful is to make all roadmaps publicly available, except for sensitive inforamtion. And then calibrate it upwards. So you as a manager, review with your manager your directs roadmap. You don't have to do this sync up often, but just enough so they can say, "Hmm... Tim's level 3 is doing much more impressive stuff, we probably need to sync up on this". And of course, with everything public you can get feedback from anyone at anytime -- but this happens less often than you might think.

In a big company this will probably lead to discrepencies across orgs. But honestly that probably happens anyways.

That's interesting, I do like the personal approach, but I'm not sure I see how you know to say, 'okay you've progressed enough that you can now call yourself a Senior Software Developer'

Ideally, I think I'd prefer not to have to assign labels at all (for the very good reasons outlined in this thread), but in practice I've found this not to be practical.