TolFun is a lower bound on the change in the value of the objective function during a step. If |f(xi) – f(xi+1)| < TolFun, the iterations end. Solvers generally use TolFun as a relative bound, meaning iterations end when |f(xi) – f(xi+1)| < TolFun(1 + |f(xi)|), or a similar relative measure.
Coming to your question how to choose an appropriate value for it geneally when doing curve fitting , TolFun is not compulsory argument while using the fit function . Be default it takes value of 10^-6 . Try fitting the curve with the default value and See what are your results