The easiest way to mimic this and the way I do it in pistol and knife courses is to have someone stand about 7 yards from you. As they start walking towards you, draw. As you get better have them walk faster and faster until they are charging you. Then add a training knife or baton. This is best done with a Blue Gun or an air soft and the required safety equipment. You can also change it up by sitting in your car, or just standing in the open door of your car. You will see that your body movement to the side or up and down will may have a major effect on the mechanics of your draw. When someone is charging you your mind is beating you over the head to get your hands in front of you. This will become even more evident as they produce weapons. You can have your training partner change out weapon. If they are charging you and you don't see a weapon, do you necessarily want draw your gun?

Be honest with yourself, you may need much more practice or even a change of holster or carry position.