The term RECKON WITH 'UP' can be broken down into two crossword solutions: SUM and TOT. SUM is a three letter word meaning to add UP or calculate the total of a set of numbers. TOT is also a three letter word meaning to count or calculate the total of a set of numbers. Both words are used to describe the action of calculating the total of a set of numbers, but SUM is more commonly used in mathematical contexts while TOT is more commonly used in everyday language.