It'll all come down to:

1. Could you have reasonably escaped?

2. Could you have reasonably de-escalated (or were you the one who escalated to get here)?

3. Can you convince a jury (and the cops) that your life was in danger? If there are no witnesses, this is tough. Typically you're allowed to use deadly force only if you fear loss of life/limb. Yes, yes, plenty of cases where juries ruled in favor of the shooter when there was clearly no risk of loss of life/limb.

4. Do you have the relevant insurance to cover your legal defense costs? If not, you'll likely make a plea deal with the prosecutor even if you were clearly in the right.

I would say if the guy lunged at you with a knife and you drew your gun and he ran away, you'll be fine if there are witnesses.