Lol it's fine, I guess, it's just weird hearing people say "how do I star KO with this move". Usually it's just, "how do I KO with this move".
But anyways, it doesn't really matter what percent your opponent is at to go for the KO, if you have them mega close to the top blast zone and can pull off a Tornado, then do so, don't worry about percent, because by the end of the Tornado they'll have enough to kill them.
Really the best way to do so is hit with 3-5 (depending on char weight) then finish with Nado while they're high close to the blast zone, it's actually easy and hard to KO with it, because they either say in and it's gonna happen or they get out early and you're in a bad position. But Nado kills off the top really shouldn't be your main concern.
I suggest reading the guide I posted, it's stickied on main MK page.