Like calculator, where nowadays everyone’s so used to them people have became pretty bad at math in head.

Were people ever very good at math in head?

There are those who have become calculator dependent who might not have if there were no calculators, but I'd say they're a small middle ground. Some people are still good at math in their head, and even when they are, they should be using a calculator when it's available to double check their math when it might be in question.

At the lower end of the scale, there are people who never would have been able to do math in head, but with calculator can do math all day without problem, except when they mis-key the question and have no idea that the answer is wrong, because they have no sense of math without the calculator.