Hi Pardal
Here is a formula you can use.
You should know the wheel diameter and then base on the fan model (forward, backward) you can choos constant coeficient.
R=R0 * (1 + K * Teta)
R0 = wheel radius
R = Shell radius at teta angle
Teta = angle (radian)
K=0.0023 for backward and K=0.0021...