The percentage error which I found out =[(analytical value  finesse value)/analytical value]*100
But inorder to find the finesse value, I just used curser to get the central frequency of each peak and by substracting one from the other I found TMS and FSR.
The resolution was 6500 Hz. Thus, it seems that this method is not actually reliable. I am trying to find the central frequency of each mode with the help of lorentzian fits. I am attaching a fit which I did today. I have plotted its residual graph also.
I am uploading 4 python scripts to the github.
1. Analytical Solution
2. Finesse model cavity scan
3. Finesse model fitting
4. Finesse model residual
Quote: 
Hmm? What is the definition of the percentage error? I don't obtain these numbers from the given values.
And how was the finesse value obtained from the simulation result? Then what is the frequency resolution used in Finesse simulation?

fitting_1.pdf 