My chamber is about 2" above the keel at the transom, and angles slightly upwards relative to the keel. When on plane, the back edge of the chamber is about 2" out of the water. When you build one, make sure it wont drag in the water while on plane.
Great info Denny
Bits, if Denny's cavitation place height is 3 1/2" w/ 30" setback, I would think 3" w/ 24" setback is a good place to start.
Good luck!
Lloyd