Yeah SDM's frame data is wrong in places. Fox starts the side b in an animation the game simply calls "304". The cancel frames are the same frames that fox is actually moving/has hitboxes, an animation called "305" which lasts 4 frames. After the movement fox enters the animation named "306" which lasts 40 or so frames or until he hits the ground.
Pressing B during the 305 animation skips it directly to the 306 animation. The 306 animation is the same regardless of the shorten; fox moves forward a slight amount, the same amount as if you cancel the 305 animation before the first or second frame.
And yeah, the pic isn't perfect, but I don't really care (dealwithit.gif). I could do what you said, but I don't have any motivation to do so tbh. If someone wants this, PM me about it at a later time and I probably will do it.