Anyone who tries to change the system will have to 'reckon with' me. = Anyone who tries to change the system will have me to 'reckon with'. [=I will oppose/fight anyone who tries to change the system]
She 'was reckoned' [='considered'] to be among the group's leaders. = She 'was reckoned' as one of the group's leaders.
When he won his first three matches, the other players realized that he was 'a force/man to be reckoned with'.
(..) know what they want. I think there's only enough change for one of us. You have it. You don't even look like him. I reckon I look like him more than you. I reckon you should take the couch and I take my bed back. What do you reckon? Yeah, I reckon that's the best idea for both of us. (..)2014 The Dark Horse