Simple-Fit
This Simple Fit app provides a convenient way to perform curve fitting. With the data plotted, simply open the tool, type the desired equation, specify initial parameters and generate your report. You do not need to even save or create a fitting function.
y0 + A/(w*sqrt(pi/2))*exp((-2*(x-xc)^2)/w^2)
Then define the values for the parameters: A=90, xc=25.