storey and a half means that the sidewall hieght is approx 12 ft, so at the sidewall you have full ceiling downstairs and half hieght upstairs. doesn't matter how much floor is covered on the second floor.
I would think that as you increase the hieght of the second floor, your downstairs braces would get longer, and i think we are talking in the 1/3 of post length here. for example, a 9 foot cieling would go with a 36/36/50.91 brace.
It's good to have some of the main floor open to the second floor, I prefer to get that open area in conjuction with the staircase/chimney rather than leave half the second floor open to below.
