I think something big is coming, something unexpected during Wano or something like this. We really can't predict One Piece completely since every time something unbelievable happens. Like the start of the WCI with Sanji or Ace's death during Marine Ford. Therefore, something new can be there and make it reasonable that why only 20% of One Piece is remained. I agree with @Admiral Ryokugyu that Oda can't accurately identify some numbers like that. And there must be some slips here. But still it's what Oda thought in that moment and it's close to the real number, though still with some mistakes in reality. If exactly 80% of One Piece is done, then we got like 183 more chapters, approximately we can say it's another 200 chapters and then One Piece is going to end. The longest arc of One Piece was Dressrosa with 101 chapters, as all of you guys know. And since we'd heard about Wano even in middle of One Piece, and according to the words of Oda himself, I can say it's gonna be really a great arc compared to the others, it's gonna make its chapters equal with Dressrosa or even higher than it. Let's say Wano is going to be around 100 chapters, and it sounds reasonable in my opinion. And now after Wano, we'll get 100 more chapters. Straw hats can steal the Red Poneglyph from Kaido and now they can have 3 Red Poneglyphs, hence only one remains. It can be hidden in Elbaf or any other island in the new world. But it makes Straw hats really close to the end of this way, since they need to have only 1 other Red Poneglyph and we will be at Raftel. Maybe that Loki and Enel theme at Elbaf will happen and Straw hats will go there and would have the 4th Poneglyph at that time, then it's the end, they will move to Raftel. It's my guess here, I don't know how Oda is going to split the 100 chapters between Elbaf and Raftel, or how the details are going to be. Another thing that I want to mention is that, there are many characters, and groups in One Piece. Like Shanks, Revolutionary Army, Black Beard, Big Mom, Kaido, Gorosei, Im- sama, CP, Marines, Shichibukai and etc... It's really hard to put all of these characters in the next 200 chapters, it sounds really impossible for me. So there might be 2 things here: Something unexpected is going to happen and all of these characters somehow will be connected with each other during next arcs. And the final blow will be there, Raftel! Or... there's gonna be like 50-100 more chapters, so we're gonna behold 250-300 chapters, instead of 200. It's possible, it's not acceptable to expect that Oda's numbers are completely accurate, but still we can guess the range.