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'.
(..) Four floors up. That's why they think they're safe. Put a chain across the door, bolt it shut, think they're impregnable. They don't reckon for one second that there's another way in. I don't understand. Dealing with a killer who can climb. What are you doing? Clings to the walls like an insect. (..)2010 TvShow: Sherlock Title: The Blind Banker Season: 1 Episode 2