Own Formula in Sigma Plot

#1
Hey all together,
I am new in this forum and also pretty new to statistics generally and hope to find some help with the results of a lab work I just did.
I am currently doing the numbers for my bachelor thesis and I struggle hard doing the fitting of them since we never learned something like this in lectures...
I measured a row of waters with 8, 10, 12 and 14% added ethanol to determine the effect on the measured isotope ratios in a laser mass spectrometer.
Now I do have the numbers and a Paper suggests me to fit the numbers to a formula of the form
f(x) = a * e^(b*x) - a

Now when I edit the already existing exponential regression models for fitting in the dynamic wizard in sigma plot, I just add a "-a" behind the normal 2-variable exponential regression. Now I wondered if I have to modify some of the other entrys, like parameters or variables, as well. My results are not the best with a given R² of around 0,76.

Help is very much appreciated, because by now time is running and I´ve been struggeling for days trying to get this right.

If you have any other suggestions besides using sigma plot and can tell me how to get it done, also appreciated!

Greetings,
Nicolas