You're probably refering to air 2K? I'm not very good at this link, but you should probably space it so that the last hit will hit your opponent as close to the ground as possible. The best idea would be timing it so that it hits only once, since the first hit stuns more than the follow ups. This is only in theory though, I don't know if it's even possible to make it hit only once without RC. According to frame data, j.2K is +8 if performed as low as possible so that's probably what you should learn to do.
Also 2P comes out 1 frame faster than 5K. Though the difference is little, it might make your combo work. 5P would come out 2 frames faster than 5K, but it probably won't connect on many crounching characters.