To get from 200 CH with fill water to 550 CH after evaporation and fill, it would take 550/200-1 = 175% of your pool water volume to achieve that. That's an awful lot of evaporation, but perhaps is what is happening in your case. If you have a bucket of water next to your pool, you can see how quickly the water evaporates. Assuming a 4.5 foot average depth for your pool, you would need evaporation of 4.5 * 1.75 = 7.9 feet of evaporation per year or 0.26" per day (actually double that or around 0.5" per day since there isn't much evaporation in the colder months).

Richard