Board speed is an odd, single and not enough parameter to describe water start features of an HF.
There are so many other parameters in the water start equation, that referring only to board speed is difficult to measure and not enough.
Under the limits of light wind everything thing is highly influenced by a mere 1 knot lull.
The kite thrust can decrease between 20 to 30% in the middle of a water start.
Because of that, the way that works for me is water starting like the wind is dying on me on every start trial.
You have to take into account that the wind window is significantly smaller than with normal winds, so you have to point the board quite downwind.
I start with my body leaning towards the front of the board (while pulled by the mid downstroke of the kite), keeping the wing aft.
That gives me an extra length to 'swing' the wing forward with the legs, giving it an extra speed for the start.
While the wing gets the speed (not your body) you try to fly so the weak thrust from the kite is enough to maintain the tack.
Just to emphasize the "trick", you increase the wing speed by striking the mast with your legs, while your body is forward.
I hope I managed to explain how to gain some "Take Off Speed" under very light winds.