The sensitivity or probability options can be used to perform a back analysis. In the case of a sensitivity analysis, you can set the possible magnitude range for the parameter of interest and run the analysis, then pick off the FS=1 value of the parameter.
For cases where there are multiple parameters, a probabilistic analysis can be used. Set the parameters to have a uniform distribution with the possible range of magnitude for the parameters. Then look at the scatter plot of FS values and look at all the possible options for values that yield a FS=1.