I think I would start with a maybe clearly "too fast" number than a "too slow" number. In the end it will still be a light boat. How much sail area will it have with the main and jib? will it be anywhere near the 20 sq meters that the original main only HT rig?
I like Jake's thinking though, but might start at 66 or something.
Come to think of it, I didn't really factor in boat weight when I made that 80/20 assumption...I would put it closer to 70/30.
