I think pravi is more like the state of being true, as opposed to false, while gxusti is more akin to being proper, sort of like being the right one as opposed to the wrong one.
As a side note, I've been trying to work out the difference between korekta and gxusta. I think korekta and gxusta can be used interchangably, but the impression I've gotten is that the connotation is slightly different. What I think is that gxusta is closer to the English "proper", meaning there are choices and this is the best, whereas korekta is closer to the English "correct", meaning there is one perfect answer and this is it.
I believe "korekta" means "related to corrections/correcting", e.g. "korekta laboro": work where you are correcting someone or something. It's not about the thing (in this case, the work) itself being correct. Mind you, it is used that way occasionally, but Vortaro calls this usage "evitinda" (to be avoided).
prava = right (e.g. someone is right)
ĝusta = correct, right (e.g. it's the correct/right answer)
senerara = without error = right
korekta = related to corrections/correcting
In this particular sentence, the adverb placement is irrelevant because it is apparent what is modifying; however, sometimes placement changes meaning. Read this: http://donh.best.vwh.net/Esperanto/Notes/note03.html