It's not a timing thing (kinda), it's a positioning thing. You actually have to DC off the first hit of dp+A. You also only get a few frames to do it in, which is why it's so tough.
There are two ways to do the dp+A to qcf+C/AC combo. You either do half-circle back, forward + A, qcf+AC or dp+A (the exact motions, do not hit forward twice), then quarter circle forward.
The second method is confusing to some, but go into training mode and turn inputs on. When you do a dp and let go of the direction, it takes on an additional forward. If you tried to do qcf+AC at that point, it'll do super. If you do not let go of the stick, it won't register forward, but will be held at down-forward. From there, you go to down and do a quarter circle forward instead from that point.
The other way, which is harder is do a kind of double-tap motion after doing dp+A, f+AC. You have to tap the buttons almost immediately after doing dp+A. Some people have success in pressing buttons than doing directional inputs, so this method works. I have never done it outside of HD mode (Where the cancel window is much bigger) so I can't speak for it.
