OK, did some testing with this. There are a few simple rules that must be followed, and then this becomes just as easy to hit as gentleman to knee. (Not easy, not every time, but oftentimes)
From what I can tell, there are quite a few different strength hitboxes in this move. You can combo into knee with all of them except the first one that hits away.
There are 2 more that I have seen. The middle, and the very end.
The middle one is about 1.5 cfalcon body lengths away from the starting point. With most DI, you can FKK (falcon kick knee) fox at 135-140. Falco percentage is about the same.
The ending hitbox is very weak, and can combo fox/falco at around 190 into a knee.
When the falcon kick hits offstage falcon does not go flying forward like he usually does. He pretty much stops. Offstage I was able to hit a the ending weak box directly into a knee. No jump. I died of course, but here is how it went. Falco side b starts, I start falcon kick. I catch him with the very end of my kick and he basically goes nowhere and starts to fall. I fall at the exact same rate, start a knee during my fall and hit him with it. Looked like a dropzone.
I have also discovered the ultimate in situational yet completely plauseable FKK combos. Percentage and DI is iffy on this one, but fox needs to be at about 125-130% this time. Fox is standing in the corner of the fin. Falcon is standing at (or a bit behind) the junction of the grey plane body and the black jet. He falcon kicks, hits the wall, does his jump and flip. You then start a knee as soon as possible as you fall, and if everything goes as planned fox falls right in front of you the same moment as your knee comes out. It works folks. And better yet when you hit that knee, they will bounce off the fin and you can walljump off of the fin and knee them again. (they probably will not be able to tech because they were trying to tech the ground earlier and will get screwed over by the 40 frame rule).
That is the best I can do. Enjoy, and try it yourself =]