Again, this assumes cover is even available and you may have a chance to reach it.Move to cover, don't worry about whether it's to your left or right or to the perp's left or right... It really doesn't matter, IMO....
You're better off moving to cover, no matter what direction, than hedging your bets on whether the perp can track to the left or to the right....
As has been mentioned, if the attack comes at less than 21 feet (or more like less than 10 feet), what do you do then?
Remember what we are talking about here: Gunfighting. Using a handgun to save your life or someone’s life you care about. It may (probably will) happen so close that you have to fight off the attacker (or control his weapon) before you can even access you own.