Merriam-Webster's Learner's Dictionary
informal: to think or suppose (something) : to believe that (something) is true or possible
▪ I reckon that we'll have to leave early.
▪ Do you reckon you'll be able to go to the grocery store after work?
▪ We'll have to leave early, I reckon.
▪ Do you think it will rain? (I) Reckon so. [=I think it will]
spoken especially British English to think or suppose something
reckon (that) Do you reckon he'll agree to see us?
The police reckon that whoever killed Dad was with him earlier that day.
'There's nothing we can do about it.' 'You reckon (=used to express doubt or disagreement)?'
Never say "Recokon about that!" in place of "Think about that!"