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'.
(..) Dialogue: Marked=..,Default,NTP,!Effect,Well, some say he died. Dialogue: Marked=..,Default,NTP,!Effect,Codswallop, in my opinion. Dialogue: Marked=..,Default,NTP,!Effect,Nope, l reckon he's out there still... Dialogue: Marked=..,Default,NTP,!Effect,...too tired to carry on. (..)2001 Harry Potter and the Sorcerer's Stone