Hey bargearse
You can also do them at very low speeds hooked in..
15knots will do it.
Kite at 11 use a little pop (2ft) do your backroll at 1/2 the normal speed or you'll over rotate &
commit to pulling your backhand in with the bar at 'full power' (dont sheet out).
If its 15knots - a 12m will be fine.. as the kite dives down, it will shoot you out the other Half of the back roll sending you up about 6 - 10ft high and about 20ft downwind if the timing is right.
Should you find yourself too high - either kick the board away or land with the back heel corner of your board to break the water first
Definately fun when youve got it down

[}:)]