Takes a lot of balls to do it man! I remember the first time I did it on my own in concrete or in a hall full of people..lol. DO NOT STACK IT. That would not have been cool at all eating the floor with 100+ people in the room!
Your leaning too far backwards which increases the distance between your knees and your head. If you jump bolt upright and lift your knees up you will rotate backwards properly. A classical backwards summersault you take off, rotate and land exactly on the same spot. Most people don't do this as they don't jump up properly, they lean backwards. Of course you can arch it for artistic merit...or for stunts.