Calculation of the Seperation factor of Ethanol / Water rectification based on two saturated vapor temperatures

I want to calculate the efficacy of a Water / Ethanol rectification based on two temperatures with help of a vapor-liquid equilibration diagram.


The first temperature is measured right above the liquid.


The second temperature is measured right before condensation.


The rectification factor doesn't has to be extremely exact.


The calculation is carried out by a computer. The curves of equilibrium diagram is calculated by regression based on a few data points.


[![to clearify, where the temperatures are measured](https://i.stack.imgur.com/S1nES.jpg)](https://i.stack.imgur.com/S1nES.jpg)


[![a simple case where it's obvious what the rectification factor is](https://i.stack.imgur.com/PP9O3.jpg)](https://i.stack.imgur.com/PP9O3.jpg)


[![But how do I calculate the rectification factor in this case?](https://i.stack.imgur.com/vatyS.jpg)](https://i.stack.imgur.com/vatyS.jpg)


