@Robby:
You're trying to make an estimate on WRITING by using MATH. Writing that constantly grows in the telling. And that is just NOT how it works. With any writer ever.
Really flawed loose variable math with no definite numbers that has changed several times over the years.
I'm pretty sure for instance at one point Skypeia was marked as the halfway point of the series, but damned if I remember what random Jump Festa or interview that came from 10 years ago.
And I know as long as I've been on this forum, it's been guessed and estimated that OP has about 10 years to go. For at least the last 8 years.
I remember before the entire Thriller Bark/Impel Down/War thing started people speculating if it would end at 800, since they were about to reach Fishman Island and that was clearly the halfway point. (And those three events were nearly four years that no one would have guessed at before they happened.)
Sure, Naruto clearly decided at some point to end at 700 and go into a movie and all, and padded itself accordingly to reach that number, but it probably decided that around chapter 670 or 680… a few months before the end... not at chapter 300 ten years early.
I may be wrong, but I seem to recall Oda mentioning in an old interview or SBS that as early as the end of Alabasta he thought he was halfway finished. If not, it was probably Skypiea. I definitely remember him calling Enies Lobby the halfway mark. Barring the series ending in the next 60 chapters, I think it's safe to assume he underestimated.
Bottom line, Oda is factually horrible at estimating just how much story there is left to tell. His editor can't possibly be any better considering he has even less of an idea than Oda himself. 70% is an arbitrary guess and judging by numerous past estimates on Oda's part, we can't reliably trust any such estimation.
Let's say this estimate is accurate and call chapter 800 the 70% mark. That leaves 342 chapters to finish the series at chapter 1142. If we're sticking with the mathematical formula approach to story planning, that mean's Oda's previous estimate around chapter 430 of being halfway there was 33% error. If we apply the same margin for error in this 70% estimate, the series would actually end around chapter 1518 exactly. So if we're sticking to math, that's a more reliable estimate as it accounts for the percent error in Oda's estimation ability. We'll just call it 'instrumentation error'. Oda-bot's brain doesn't process numbers as well as should be expected.
For as good as Oda is at estimating the duration of his story he might as well just say "ohhh we're halfway there. Oh oh living on a prayer. Take my hand, we'll make it I swear!"
With Naruto….I actually feel like Kishimoto really just got tired of writing and decided to rush an ending. Not to say that the entire last arc didn't feel like filler...it felt like filler, but I think that is because instead of giving Madara and Kaguya their own arcs he simply condensed everything into the final battle. Literally 1/7 of the entire series is the final battle which had not one, not two, but three shifts in primary antagonist from Obito to Madara to Kaguya to Sasuke, several flashbacks, and multiple power ups. I think he probably originally envisioned a more organic progression of story from Obito to Madara to Kaguya, but grew tired and just said, "Screw it, the rest of the story will be put into this final battle." Not even the entire war arc....just that final battle. If we extend it to the entire war from volume 50 to 70 then Naruto got like 3 new forms within that single arc.
I always felt Sasuke was meant to be the final villain, but instead of a fully realized arc we got that random final battle between Naruto and Sasuke that was suddenly thrown in after the defeat of Kaguya. The original plan probably called for like 3 more major story arcs, but instead we got a string of battles with every character that ever appeared in the series present to watch....so it's sort of like he took everything that made the Namek Battle against Freeza seem never ending (in the anime) and amplify it tenfold. He may have sped up even more to finish at exactly 700 within the time frame you're suggesting. I think at some point during the war he just decided he wanted to end the entire series rather than continue on to further story arcs. Might be wrong, but the three consecutive final villains in one final battle which conveniently allowed for arbitrary change in venue seemed like story compression.
Ultimately unimportant, but I think it furthers the point that Kishi unnaturally tried to pace the series to end at chapter 700, be it through filler in the final battle or unnatural compression of the story ideas he had in his head into a single battle.
Also, to clarify an earlier point. I don't consider 'Yonkou' a title or designation. I don't think Luffy will necessarily take someone's place. I just meant to say that he might reach that level of renown or influence at some point in the New World.