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'.
(..) It's too late for that. Is this normal? God, the smell. He's cooking now! They cooking him good! They're about done, I reckon! WHARTON: We're gonna have a hoedown! He's cooking now! We gonna have a hoedown! 'Cution gumbo. Give me some! (..)1999 The Green Mile