To answer the OPers question, "then what?": Most, if not all of the self-defense authorities will tell you distance is your friend, as is cover.
If you choose to present, rather than shoot, move away.
I would be inclined to present and back away myself if the situation seemed to permit it with a knife robber. Or maybe move quickly laterally towards a parked car or something while drawing and presenting
By not actually attacking, the knife robber has put himself behind my
If I can avoid actually shooting someone, I also avoid the police investigation, the possible arrest while police sort things out, having my gun seized, legal bills, and legal jeopardy if the prosecutor decides maybe it wasn't a self-defense shooting.
While some criminals might be so hopped up on PCP or meth they think my presented gun can be ignored, I'm guessing that the majority of them are plenty scared of holes in their skin.