The root word for "calculation" is "calculate," which comes from the Latin word "calculare," meaning "to count" or "to reckon."

add up, total, reckon, calculate, compute, tally, number

consider, judge, regard, deem, rate

calculate, total, count, reckon, figure out, add up, tally

To reckon means to count. Its use as a synonym for "to think" in general is non-standard.

"Reckon" actually means "calculate", although in Western American slang it came to mean "guess" or "believe".

"I reckon that his estimate was the most accurate." or "We'll have a gunfight at dawn, I reckon."