The tool will build a fillet surface, and then cut back (Face) the slab surfaces in one operation. It is not necessary to find the theoretical intersection of the slab surfaces before using the Surface Fillet tool (though you may choose to do so for quality purposes).
Construction history means that you can build the fillet surface and then modify any of the settings in the option window, and the fillet surface will update (hit the Update button or set Auto Update in the options).
