Evaporation velocity of ethanol: adding ethanol vapour to a gas mixture
Please note that I have never done this. 

But are you sure that you want to rely on approximation/assumptions?

I realize that it needs additional changes to the experimental setup, but is measuring the ethanol concentration out of question? Gas sensors for ethanol are commercially available and a setup, in which they steer mass flow controllers is conceivable.

As a side note, you might be interested to see that ethanol sensors can be build in the lab: 

A Novel Flexible Room Temperature Ethanol Gas Sensor Based on SnO2 Doped Poly-Diallyldimethylammonium Chloride.