Personally I'd say it wasn't excessive as long as you have a decent grouping. And you shout "halt!" afterwards.
Seriously, this is a human rights issue, literally - I do not believe there is a human right more basic than that of the defense of one's own life. And the "proportionality" defense generally depends on some clairvoyance on the part of the defender as to the intentions of the attacker. A fellow threatening you with a knife is threatening your life, and the fact that he says he's only interested in your wallet is irrelevant even if true. If he snivels later that you shouldn't have shot him because he wasn't really going to hurt you if you just cooperated that he ought to be laughed out of court and in most sane, civilized places such as Dodge City in the day, he would have been.
You only own what you can effectively defend
And if your life can only be defended by the State, then your life is owned by the State
Unless, of course, you consider the defense of one's own children. Issues of proportionality fade into insignificance. If there is a chance that you might harm my kids, you will be STOPPED, regardless of what it takes to do so.
If I am attacked when alone, there may be an opportunity to withdraw and that may be the prudent thing to do. Accompanied by my kids, withdrawal becomes extremely difficult.