reckoner
/ˈrɛkənər/Definitions
1. noun
A person or thing that reckons, or a person who calculates or estimates the amount of something.
“The taxman will be the reckoner for your annual income tax.”
2. verb
To calculate or estimate the amount of something, especially money owed or required.
“The accountant will reckon the total bill at the end of the month.”