That would depend on the situation. In the majority off targeted attacks the person is on top of you before you can pull. Generally, and culturally, an Americans personal space is about 4 feet. Much more if there is only one or two people in the area. But that person can spring across any reasonable space before you can draw your weapon. The 21 foot rule comes from a test in which both parties knew the test was happening. You do not have that luxury.
So, what is likely to happen is that you are so close you have to use your off hand to distract, redirect, and make space while drawing with your good hand. Roll that from the holster and pound two into his lower body. Don't worry about all the center mass stuff, that comes later, but put rounds in him, anywhere in him.
Then you back out while covering him and glancing to the sides to see if any other threats are presented. Now you are extended on him and making as much distance as you can. If he is capable of chasing you and does so you then fire center mass, if not get away to safety.