In my experience it's better (in terms of using solar vs steam) to turn it off much sooner than that (maybe 15% or 20%) so that you're minimizing the amount of

In my experience it's better (in terms of using solar vs steam) to turn it off much sooner than that (maybe 15% or 20%) so that you're minimizing the amount of wasted solar energy later in the day when the accumulators are full and the base isn't consuming all the solar output. You might end up with the steam turning on and off a couple of times over the course of a night, but it'll still be a shorter total time than if you wait until 50% to turn off.