This is a very basic video, of which my interest is the diagram near the middle. It has made me question how extreme a pose he will pull whilst in the air, with the legs that dramatically high it could add good exaggeration to the scene or make it look a bit too much and silly. Again I will try this in my scene. I do like the arms over the head pose however as I used it in my first chair attempt because he is being shot at and it can be seen as a sort of protective gesture like he is trying to shield himself. If it does give him more power in his jump then it could show even more thought behind his pose.
This website tells you how to achieve a high vertical jump just like the video above. This is mostly what I could find about jumping on the internet.
Looking at pictures of jumps on Google, a common theme is to have the arms in the air and the legs bent up. I think the arms up is a bit extravagant for jumping off an armchair so I may just have a little movement in them, but the legs up is a definite possibility as it will give him more height, be a dramatic key, and also give him enough clearance between himself and the arm.
This research did make me wonder what type of chair he has to jump from?
Obviously the more cushioned the chair, the more bounce he will have from it, which will give him more height and a better jump. If the chair is just a cheap plastic one or more rigid, it is more likely that it will fall over with him rather than a clean jump off it. If it does fall over he could use it as a shield and pick it up rather than hiding behind it.
I think these are more questions for the Whack'd team, but for now I will explore the different possibilties.