Here's what it was for me.
Got sent to the OSI clinic (St Josephs, London) for testing and determination. Written questionnaires, psych evals, etc. Took close to a year for tests and full determination. Once VAC received the info, I went on rehabilitation category with weekly visits to the shrink. VAC agreed that the condition is there, just didn't know how bad. So they gave me 10% for the condition, as a start. I've been in treatment for almost three years. VAC requests updates. 3-4 page forms regularly. About once a year they check progress with a 16 pager report from your head doctor to the mental health person at VAC. They let things run, decide down the line how bad you are, what are your chances of getting better are and top off to whatever percentage they've decided your condition is rated at. Now, I don't know how that fits for you or others, but that's the way my case has been handled.
There are also the associated conditions, already mentioned, that get taken into consideration also. Hope that helps.