it kinda depends on which move you're trying to JC, but in general it's after the move hits.
dunno if it'll help, but go to training mode and turn on the input display. then do whatever combo you're practicing, and look at the inputs. if there's an up arrow before an attack button, then you're hitting it too early (over-anticipating?). otherwise, i think it's still possible to input jump after the attack button, but too early for a jump cancel to occur.