You can fly the hull in 15-20 kts steady with moderatly rough seas. Just don't be afraid to take advantage of the power the boat has. If you want more power, let off on the downhaul some. This will give you a deeper sail shape. Fall off from the wind as far as you can while keeping your telltales flying when the main is sheeted in. Keep an eye on the jib and trim it correctly to keep the slot optimal.

When I first started hull flying I used the main sheet to control the angle of heel. I would let off the main when I started to go too high. I would react to the wind instead of staying ahead of the boat. You can see puffs coming and should steer the boat to adjust for them. Fall off when the gust is gone and head up when the gust hits. This keeps the sails "powered up" and doesn't have as drastic an effect on the heeling angle. The boat doesn't react as fast when steering to control the angle of heel, and it will take some practice to get used to it, but you will find you can keep your hull up longer when you get good at it.

I hope this made sense. If not someone will chime in a clear it up, I'm sure.