I would suggest looking at this a different way. Your foils, that is your boards and rudders, are your "underwater sails". And just like you can benefit from more sail area in very light winds, I believe you can generally benefit from more foils at very low boatspeeds.

Hence, in very light air, I keep all of my foils down all the way around the course. As boat speed goes up I consider reducing my foils. So in medium to medium+ I might start pulling my boards part way up for downwind. In really heavy air I might pull my boards part way up and have them part way up all the way around the course.
The way I look at it you need more foil in light air to prevent stalling the foils (very high drag) and at low boat speed the surface drag from the foils operating with good flow is minimal. At greater boatspeeds the foils will not stall (so you need less of them) but the surface drag begins to increase significantly (so you pull them up) reducing surface area and surface drag.