Personally I'd want more space around the tub than that, because if you ever DID want to sit around outside the screen room, you'd be much more comfortable.
I also think asthestically it would look better if the deck/tub were not symmetrical- maybe have only 1.5-2 feet of deck on two sides of the tub, but have quite a bit more on the other two sides.
Do you want room out there for a fire pit? (not on the deck, obviously) BBQ? Plants?
Anne