Bracket placement question

[take a potter]

I am beginning to build the mold for a bracket for my 23' cc. I intend to build a "Hermco" style twin bracket, so no rise front to back of the keel of the tub.

My question is how far above the boat keel/transom does the bracket need to sit. I have seen everything from 2-5" referenced. By my calculations the bracket will displace 1500 cubic inches, so about 55 lbs per inch of depth at the waterline of the tub. I would like to have the bracket tub 3" above the keel/transom, because I think this will allow water to pass, for the most part under the tub at speed, eliminating the trim tab effect. Does this sound appropriate or do I need to go higher and loose some flotation? One plus to going higher is that I would have room for the transducer mounting brackets under the tub. At 3" I will build a small pocket into the bottom leading edge of the tub for the brackets.