With the Carve 144 a good compromise size would be 46 cm, possibly 42 for the smaller sail and 48-50 for the 9.0. That is about the sensible range. Bigger fin for lighter winds and bigger sail, smaller fin when well powered up with the 7.5 sail.
They Hypersonic is quite a different design. It makes do with a lot smaller fins, maybe in the 30-35 cm range.
For best allround performance move your straps back a little, to the mid positions. That usually gives a more natural stance.