I found a formula on the following website to calculate the windsheet thickness (WST). The WST and mouth height are defined in the image to the right. I'll summarize here by saying that a scientist named Hartmuth Ising developed a number to characterize the mouth of a pipe. The Ising number should be between 2 and 3. I=2 gives you maximum efficiency. As you increase the number, you get more harmonics and anything higher than 3 will overblow. I've found that I=2 gives good results. Here's the equation for calculating the WST:

WST = (I2f 2ρH3) / 2P

    I is Ising's number
    f is the frequency
    ρ is the density of air
    H is the mouth height
    P is the air pressure

It's important to be careful with the units in this equation. Using 1.2 kg/m3
for the density of air and putting in a factor to correct for the units, the
following equation will give you the WST in inches.

WST =	I2f 2H3	* 1.554 x 10-6
P
    WST is the windsheet thickness in inches
    I is Ising's number
    f is the frequency in Hz
    H is the mouth height in inches
    P is the air pressure in inches of water