How to calculate the cell constant for a flow-through conductivity cell?
Here is a webinar on using Comsol to model electrochemical systems. 

I think in practice you'll get close making some simplifying assumptions and then calibrate it with a reference solution. 

I did some commercial conductivity instrumentation for RO systems used in hospital medical dialysis installations, and the conductivity cell response has some 'interesting' dependencies on both measuring voltage and frequency.