IMO the easiest way to control the boil w/ heating elements is to simply size the element to the boil and run it 100%. This discussion could lead one to believe that in order to boil w/ electric one needs a controller, pids, heat sinks, BCS 460's and other instruments. While not the best or most sophisticated it will certainly work running 100% as follows.
1. Connect power to proper sized element
Of course the downside here is that it will take longer to reach boil as you can't up the wattage to achieve boil and then reduce, not that big a deal IMO.