Prior to drainage (pre-1882),
The optimal Lake stage was 22 feet above sea level.
That’s the level it naturally drained south into the Everglades downstream. In modern times, the question gets bogged down in a complexity of water management schema, stakeholder clout and the constraint of the Lake’s perimeter dike. No longer allowed to spread out, high waters are drained through a release valve to protect wetlands on the inside of the dike instead of replenishing the wetlands it used to feed to the south. The primary release valve is called the Caloosahatchee River, although technically to get to the main river stem (which is actually a widened canal called the C-43), water has to first drain through Three Mile Canal. Prior to drainage the natural river ended at Ft. Thompson Falls just upstream from present-day LaBelle and 20 miles from the Lake. Yes, it’s complicated, and muddled (and muddy). It’s called modern times.
The thing about the dike, and this has always been the case:
It was built to control water on outside, not inside, its bounds.