Own Formula in Sigma Plot

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!