For me there is a set of calculations I can do without thinking and there is a set where it takes several seconds to figure the solution out. No smooth transition. Is it the same for you? If yes, this would make tuning this game pretty hard because it is either to easy or to hard.
Yeah, I was sent a link to a similar site yesterday (timed mental arithmetic/algebra), and was thinking exactly the same thing: divided by types, half the problems I could do instantly with almost no thought, but the other half, regardless of how easy, took a few seconds.
I noticed that with, for example, multiplication, I have a set of rote-learned answers for calculations that aren't 2, 5 or *10, and if the answer isn't one of those answers, I'll get the closest answer I can remember, and work back/forward from that. It's entirely habit, and I can't stop myself doing it. Algebra as well, I always have a moment where I swap parts of the equation in my head; even if it's utterly simple, there's still a noticeable cognitive step.
When I turned squares on I hit a wall at 23^2 etc. I can do it, but it's 10 seconds just to do that calc. Basically anything bigger than 14.
For the rest it was more about staring at the numpad (which I never use) and typing in my answer. If I typo'd I lost several seconds as I'd not have a finger on backspace :(
