One thing you got wrong is that it goes the other way around. Kumusta ka is the informal way while Kumusta po is the formal way. Also, Kumusta po can also be written as Kumusta ka po. There won't be any difference in the basic meaning of the phrase. The word 'na' can also be added before the 'ka', like "kumusta ka na", or "kumusta ka na po".
to an older sibling, parent or figure of authority: Kumusta ka po?
to a friend or acquaintance" Kumusta ka?