Placing main subject on the left or right?

is academic question, and you will never "set" a rule for it. Many times, after Carravagio, there was attempt to teach through set of roles, especially in England where art was at its "high". Hovewer in that way skilled painters never produced a great work, while "free" from roles always broke sich rules and made a great work.

Rules are still around in art but just for students to learn, and just after they get 'em they have to forget 'em.

And to answer your question:
Main subject should be there where you artistic instinct say, period. If you do not have such instinct leave it to the left of the viewfinder.