Nothing wrong with your math. The Mississippi is 14 ft above seas level. Lake P is 3 ft above seaslevel. Even if a levee on the Miss side goes, all it will do is send a cascade of water through NO down to the basin and the water will rise to Lake P levels.
For the water to rise to Mississippi levels would require Lake P and all the surrounding area to fill up with enough water to let that happen. And that would require the gulf level to rise accordingly also, as all the water would just run off into the gulf.
Last time I checked only one flood has measured up to thiose levels, and I believe the only person tosee it was named Noah.
Lake P drains slow, we saw that after Katrina. I doubt it varies linearly with height though. The Industrial Canal lock amounts to a small "levee breach" and wouldn't raise Lake P much. The spillway is another matter. Anecdotal reports have the spillway locked open, but the only published report says only that "a lock" is open. Still waiting for more info.