Don, you are totally right. I should have noted this simplified method is "almost exact" but not truly exact as you point out. In the end, you are usually dealing with a difference of .1 stops, something very difficult to even set a lens for, hence the reason I usually advise people to keep it simple and get close to the 1/3 stop settings the lenses are marked for.

In reality, I no longer use any of these methods. I determine the distance I need to focus to acheive the required DOF I am seeking. I use a laser rangefinder to find something at this distance, then focus on it. My DOF chart contains the exp. compensation required for focussing at that distance. It's just one less thing to think about. Of course, since its on a cheat sheet, I use the formual to do the exact math.