As someone who used to think climate change was the only issue that mattered in the world, I have developed a chronic dislike of leaving the water boiler on after the water reached the boiling point (have to painfully suppress the urge to do anything when I'm with others to not be socially weird), and also of using more water than is necessary. Other people generally just put a lot in, heat it, and then only use like half. Sinful behavior, I know.
I don't always want to go through the trouble of filling the water into the can first, but the water boiler has no markers indicating where it is one liter (which is exactly how much fits into the can). So I did the reasonable thing, which is train myself to know exactly how much one liter is.
However, instead of mastering this skill, I instead developed a habit of caution. Just put in a safe lower bound, that way I will never have to throw it away. Only when I consciously correct for this do I avoid it -- and when I do, my estimate does, indeed, tend to be quite good.