There is a natural notch a few centimeters below the tops of the deltoids- experiment a bit, you will find it...
"When a powerlifter squats with a low bar position, the bar should be placed no lower than three centimeters below the top of the anterior deltoids. For other lifters, comfort and flexibility will go a long way towards determining bar positioning. When gripping the bar, at first it is best to place your hands as close together as possible, to maintain tension in the upper back, and to avoid any chance of the bar slipping. As a general rule, the lower you place the bar, the wider your hands will have to be. Anything placed between the bar and the lifter, such as a pad or towel, decreases the force of friction and increases the chance of the bar slipping. It is to avoid injuries that this practice is banned in competition. Also, this will artificially raise the lifter’s CCOG, which makes it harder to balance under a heavy load"