there is no "acceptable" seat to bar drop, its all relative and different from person to person. you also have to consider the riding style and the application of the bike. having a large drop to put you in a more aero position for performance purposes is cool, but i wouldn't want to commute or cruise around the city on that setup. as far as excessive seat/bar drop.....when you're really uncomfortable from the position(too much weight on your hands, pain in your lower back/neck) thats when its excessive.