The "allowed fluid height" is simply a rearrangement of the required thickness equation. As you get higher in the tank, the minimum thickness is typically over-ridden by the Code minimum (for instance, the equation may yield a value of 0.23 but the Code states that the minimum is 0.25). When this happens, the thickness is bigger than the equation computed value, so therefore the allowed fluid height is bigger than needed.

I can't answer the wind girder issue without looking at the data file.