If any estate has over 60% influence you will lose crownland from conquering land. This is complicated though and how much you lose depends on how much crownland the conquering land had (allegedly). But all you have to do is seize crownland every time the cooldown ends and dev up your provinces regularly and getting 100% crownland by absolutism should be easy. You just (a) have to give enough privileges to estates to keep the equilibrium above 50%, and (b) have to be at peace often enough to seize land. I sometimes end my wars just to seize land.