Reverse the following process (Source):
Speed to Pressure Conversion
Write this equation converting wind speed in meters per second (m/s) to pressure in Newton per square meter (N/m^2):
Pressure = 0.5 x C x D x V^2
C = Drag coefficient D = Density of air (kg/m^3) V = Speed of air (m/s) ^ = "to the power of"
Obtain the wind speed value you wish to convert to pressure. It needs to be in meters per second or the equation will not work.
Example: V = 11 m/s
Estimate the drag coefficient based on the shape of the surface of your object that faces the wind.
Example: C for one face of a cubic object = 1.05
Sphere: 0.47 Half-Sphere: 0.42 Cone = 0.5 Corner of a Cube = 0.8 Long Cylinder = 0.82 Short Cylinder = 1.15 Streamlined body = 0.04 Streamlined half-body = 0.09
For additional information regarding these shapes, visit the link in the Resources section.
Plug the values into the equation and calculate your answer:
Pressure = 0.5 x 1.05 x 1.25 kg/m^3 x (11 m/s)^2 = 79.4 N/m^2
Perform any necessary conversions to the units you desire. The wind speed must be in meters per second for the equation to be accurate.
Convert mph to meters per second (m/s) by multiplying the speed in mph by 0.447. This value is obtained by dividing the number of meters in 1 mile, 1609, by the number of seconds in 1 hour, 3600.
Example: 23 mph x 0.447 = 10.3 m/s
Convert Newton per square meter (N/m^2) to psi by multiplying the pressure in N/m^2 by 0.000145. This number is based on the number of Newtons in a pound and the number of square inches in a square meter.
Example: 79.4 N/m^2 x 0.000145 = 0.012 psi
Your biggest problem should be determining a useful drag coefficient. You get to pick one, though I'd suggest using the value of a long cylinder (0.82). Since your wormhole/portal is a fictional object of your own creation, you can choose whatever coefficient you want.
Once you've done that, you can use this resource to move back and forth between CFM and MPH.