I always use the calculator.

But, because the numbers that get returned aren't always the right numbers, I try to approximate the answer in my head or with paper and pencil to kind of make sure it's in the ball park.

Also, sometimes it returns digits that don't actually exist, and it's pretty insistent that the digit is correct. If I catch it early I just re-run the equation but there is a special button where I can tell it that it used a digit that does not actually exist.

Sometimes, for complex ones, it tells me it's trying to calculate and provides some details about how it's going about it and keeps going and going and going, for those ones I just reboot the calculator.