I tried 65% at the time of that interview at the going rate of 10 episodes per volume and 50 volumes was close to the number I got, but it does seem short. Hope that made sense.
That's sound math, but I doubt Miura himself calculated it like that. It was probably more of a shoot from the hip kind of thing. Going at the rate Miura has been progressing the story, I just don't think everything can be wrapped up in 12 volumes. It's not a perfect analog, but consider that 12 volumes ago, Guts had just gotten the Berserk Armor.