Aside from trying to avoid creating a snow plow, I don't think there is a simple answer to the height question. From an aero perspective, I
think that "lower" is better, but only up to a point. Drag forces are the product of your drag coefficient and the
area. Your gains from a dam will be realized by reducing the Cd. The penalty can be from an increased A (and hitting stuff). Getting below the lowest point on your suspension is a given, but others,
particularly AndrewJ, here have proven benefits by extending even lower.