ANSYS has already done this for you.
In Mechanical, with the Solution branch selected, the Context Menu is Solution and there is a Toolbox button. Under that is a Stress Tool. Once you insert a Stress tool into your results, you can configure it for Max Tensile Stress which will compare the Von MIses Equivalent Stress to the Tensile Yield Strength of the material, and the Safety Factor plot will do the equation you want.