A reckoning is a calculation or number you estimate. You might say, "By my reckoning, there are now seventeen kids in the bouncy house, which might be a few too many."
The act of counting or tallying can be called a reckoning, and so can a person's opinion on something. You could describe an ancient astronomer's reckoning that the Earth was at the center of the universe or your own reckoning that guests should never stay more than three days. And if you and your estranged sister have a reckoning, it means you finally get together and hold each other accountable for past actions.