If you're just fiddling about, ignore FTSD. If you need a particular magnification, that will give you a working distance.
If you want exposure factor, that's from the film to the ~center of the lens. At 50mm extension with a 50mm lens, that's 1:1 or 2 stops/speeds.
For my casual(that's all it is) style I don't calculate it. I know 1:2 is one stop and 1:1 is two stops. In anywhere between I fudge. IE:1:1.5=~+1.5-1.6stops.