If you can come up with an outside referent for you claim that something is "easiest" or "better" then you're welcome to it. Preferable in metric units.
You're welcome.
Will empirical study of the cognitive load incurred by various forms of mental arithmetic do - using speed of calculation as a proxy for ease?
Because that's something that has been studied. If we are really picky one of us can likely search up a reference or two. But broadly, iirc, the order of rising difficulty is mental addition, subtraction, multiplication, then division. This translates to rising scales, in which the typical action is adding some small number, being more intuitive to humans than falling ones, especially if you get into negative numbers. Smaller ranges are simpler than large ones, etc.