From Longman Dictionary of Contemporary Englishreckon something ↔ up phrasal verb British English old-fashionedCOUNT/CALCULATEto add up amounts, costs etc in order to get a total SYN calculate Pat was reckoning up the cost of everything in her mind. → reckon→ See Verb table
Examples from the Corpus
reckon up• But as for the Manson Classic S, I reckon it adds up like this. 