There is no hard and fast rule about this. Everyone works differently, everyone understands things differently, everyone has different strengths and weaknesses. There is no magic number. I will add that you shouldn't be aiming for 40%. If you aim for 40% and miss it, you have to resit. If you aim for 40% and get something in the 40s, future employers will wonder wtf you were doing. If you want to give yourself an easier ride for finding work later, try harder.