Well according to explanation given to Luffy by Crocus as they enter Grand line, there are 7 ways to sail through Grand line by following the Log Pose. Then of course you can add the possibility to change roads thanks to an eternal Pose. Whatever which way is chosen they lead to Raftel still some are shorter (less islands on the way) than other. I believe the middle way (put in green in anime for whatever reason don't remeber it beeing specifically put inrelief in the manga, anyway…) had something like 12 islands (I would need confirmation on this one as this was a long time ago) so if you count Wiskey peak, Little garden, Drum island, Alabasta, Java and Skypea, you're indeed halfway down.
I do not think however that Raftel will be the end of the journey for the crew as they'll probably have to do something else once they found the Island. Several theoris (well, rather assomptions in fact) exists on what they'll find on the island, some believe it's the one piece, other the Rio Poneglyph. Some may even think the one piece actually is the rio poneglyph. I would personally doubt that as it would end Robin and Luffy's quest at the same time, assuming the Rio Poneglyph is the text of history as many think it is, including Robin.
Anyhow I don't think Raftel is an end as such. It was told that Logetown is the point where all starts and end (or was it started and ended?). Anyway I still think they will go there as it placed at the end of grand line as well (though in East blue and not in Grand line self). Couldn't logtown actually be Raftel, nobody would know but Roger anyway?
Other thoeries would be to end the quest on Red line which would also be a nice idea. We know very few about the continent and did not see anything of it but reverse mountain. There has to be a safer road to Grand line than Reverse mountain anyway.
Thus maybe halfway down but still way to go, Luffy's position in Grand Line does not have a lot of influence on how long the story still goes anyway as some arcs (read islands story) may be way longer than other.
