I think it is possible to use too much leg drive on push presses but, your leg involvement all depends on how low you dip prior to your push press.The whole point of this exercise is to overload the muscles responsible for pressing weights overhead, in theory, the more weight you can get overhead, the better.As far as form, I can only offer one more point from my experience:Once you blow the bar off your shoulders using leg drive, you can try to move your elbows in closer while you do the press portion.I feel this allows the triceps to become more active and has helped me elevate greater poundages.