Modifying a soldering iron for lower power
Ditch it and buy a temperature-controlled iron. Even at lower power your iron will keep heating if you leave it unused for some time. This will go on until there's an equilibrium between generated heat, and heat drained to the environment. Problem is that at low temperatures little heat is drained, and the temperature will settle at a too high temperature. Most likely making your first solderings bad. While you're soldering sufficient heat is drained to the PCB and your components, and the temperature will drop, so that only after a few seconds you can solder properly.
A temperature-controlled soldering iron will reduce the power when the iron isn't used for a while, thus preventing thermal runaway.  

In theory you can make your iron temperature-controlled, by placing a thermocouple near the tip, and switch the heating element by a triac, controlled by a control loop. I would just buy one, though.