Earlier quoted context omitted.
About Germany, §32 StGB doesn't require proportionality, the test is "Erforderlichkeit" or "necessity". So if you only have a gun and get attacked by a knife, you don't have to "make things even". Of course, if someone is simply stealing a Catalytic Converter you can't really shoot them, as it's a bit much... you can point a gun at them, and possibly even shoot the air, that's easier to argue as self-defence. If they…
§32 StGB requires "Erforderlichkeit" (necessity) AND "Gebotenheit" (appropriateness). The knife vs gun example is from legal education to teach that §32 StGB requires the mildest means available in the concrete situation.
And there are even other articles like §33 StGB, which excuses excess arising from fear, fright or confusion, which is what happens if someone points a knife at you.
"Gebotenheit" refers mainly to what I'm talking about in my second sentence (trying other means before shooting) and to sozialethische Einschränkungen (e.g.: psychosis, provocation, trivial attacks, etc), it doesn't "forbid" using a legal gun against an assailant actively attacking you with a knife as a last resort.