Looks great at that speed!
Usually, when I'm going to seriously reduce the "juice" to the heater, I start out with it at full power, to heat the water faster, but mainly to run the engine a bit hard at start up so as to warm the cylinder quickly, then turn the power to the heater down. If you try and start out with a low heater setting, you will be encouraging condensation from introducing steam that is just barely steam into cold steam lines and cylinders, thus creating a lot of water where you don't want it.
Just a bit of my experiences, for your consideration.