Orifice sizing for a simple pump kit:

Differential pumping orifice size
Input Parameters
Housing Diameter (mm)         Pump Speed (l/s)  
Housing Length (mm)         System Pressure (mbar)  
Temperature (ºC)         Sample Pressure (mbar)  
Molecular Weight
    Orifice Diameter (mm) =   1.53
Reference Data         Mean Free Path Check
Housing Conductance (l/s) = 33.41       Mean Free Path (mm) = 670
Effective Pumping Speed (l/s) = 21.46       Knudsen Number (Kn) = 438
Throughput (mbar l/s) = 2.15E-5
Orifice Conductance (l/s) = 0.2146

The orifice sizing calculation is valid only if molecular flow conditions exist throughout.   So long as the Knudsen number is >>1 this will be OK.

Suggested criteria
Kn > 5 : OK
1 < Kn < 5 : Caution
Kn < 1 : Calculation is invalid