I want much more stringent requirements for changing CA. Wiz is definitely moving in the right direction. To take England's case, I would say that in December 1066 CA is high maybe even absolute. William is triumphant and no one can stand in his way. RR is rather high, but he is ruling the Saxons like the conquered people that they are. Things go along like this, with high authority, until the Anarchy, when it falls very low. Under the Plantagenets, it goes back and forth. The Magna Carta seeks to set Crown Authority to Low or Medium, while Edward I and the first consistent parliaments hold it at Medium or maybe High.
I think that you could figure out a timeline for many kingdoms and think about why CA was high, low, absolute, or non-existent at different points in history and try to model that as a universal idea. When you have a lot of dukes and a very small royal patrimony (like France 1066) the best you can hope for is Autonomous Vassals. It really takes a Philippe Augustus to start increasing authority by making a larger part of France administered directly more or less by the crown. Philippe le Bel rachets it up to high I think, based on being able to revoke the titles of nobles thought to be favorable to England in Gascony and of course the Avignon Papacy.
Maybe I'm being too radical here, but I think that CA should be something that can be changed by event, not simply voting on the laws.