if the assist doesn’t come out, you’re calling it to early. if the assist comes out, but it misses the opponent, you’re calling it too late.
i think the confusing part is that the timing seems to (at least appears to look on screen) be different depending on where and when you hit the j.s on the opponent. i believe the earliest you can call it is as soon as your character can freely act. which i find generally is around the time you can start to see your character on-screen again after the flyscreen. or if you do a very late j.s that there is no flyscreen, then the moment you land.
so i think you have to not only keep in mind how you time the otg assist, but also how you time your air combo.