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'.
(..) Perhaps. They're real accommodatin'. -They look it. -Listen.... Why don't you just go on upstairs and we'll reckon accounts later? -Sounds good to me. -Are those two girls machines? Now, how can you say a thing like that? Come on. What's goin' on? (..)1973 Westworld
(..) You've got a three-millisecond cycle delay which should be computed in. l don't know what to do if the stagecoach is late. Oh! lt's fantastic! l reckon that's everything. Bath's down the hall. Bath'll cost you two bits for hot water. Dinner at , breakfast at . Get lunch on your own. (..)1973 Westworld