Trying to anticipate fighting with a HG is difficult because there are few opportunities to try it IRL. It's extremely easy to get caught flat-footed.
Twenty-one feet is not a large distance. In two steps a BG can be at 12-15 feet. In two steps you can be at 30 feet. But you might have a small child you're defending standing next to you. I would probably shoot sooner in that situation. A jury would probably agree. So, it makes no sense for any law to make an arbitrary distance a rule - there would be too many 'conditions' to reasonably append.
Basically, if you shoot someone you are screwed (sometimes, even more so if you don't kill them). If you have a good lawyer and a lot of $$ you may come out of it in reasonable shape and still be living at home. For this reason, it makes sense to use non-lethal options, backed up by a HG for the gravest extremes.
So, rather than be calculating number of feet, one should be using their own feet and get in the car and leave. But, you can also use that time/distance to be accessing your non-lethal equipment.
Parenthetically, if you have your HG (revolver-no case ejection issues) in your jacket pocket (palmed) and can just pull the trigger, you lengthen the time you have to make decisions.