It is quite a bit off. The Upper Butte Creek Watershed, which is the watershed that drains into Lake Oroville, is 3200 square miles. Much of this watershed is mountainous. The amount of water that will flow into Lake Oroville is heavily dependent on temperature (what forms snow vs. what melts snow), the existing snowpack, the ground saturation state, and precisely where in that 3200 square miles the rain falls.