I've been slowly working the one arm pushup closer and closer to the ground. Recently I worked to a level below the Smith Machine bar height of 22". Now, in case you're not aware, this is not a linear route. If you do the math, the pushing force you encounter as you approach the ground roughly follows a trigonometric formula (sine or cosine). Point is- as you begin coming off the wall with your one-arm pushups, it starts to get extremely difficult very quickly, and continues to get hard until you're close to the ground- in which case the difficulty only increases mildly. There's not a whole lot of difference between pushups off the plyo box and those off the ground EXCEPT that you get full range of motion with the box. This is key. You do not get the same ROM on the ground though the push itself is a little bit harder. I believe that a fuller ROM movement is at least a part of getting stronger in the exercise. Not all, but an important part.
Now in my video, I'm obviously not holding to very strict CC form- I'm twisting and bending and my legs are apart. However, I still reserve a less difficult height pushup for perfect (or near perfect) form. So I do these, and then I also do one arm work on the Smith Machine at about 37", and I finish with regular two-arm pushups on the ground (and begin with them for warmup). Doing ONLY perfect form work is limiting in my opinion, so nowadays I work in some more difficult pushup exercises where I am allowed to spread the feet and twist, though I do try for smooth movement.
Today I finished with two-arm pushups at about 40" because my left shoulder is acting up a little bit. Staying injury free isn't too hard with this type of work, but you still have to be careful and develop tremendous body awareness to avoid the injury.