I'm not sure how this is "early"... in the OP there was already a knife in play (threat). If the knife is 15 feet away and closing, drawing the gun is an opportunity to diffuse an already escalating situation. If the BG turns and runs, the gun has done it's job... shooting someone is the LAST thing I want to do, even beyond the legal ramifications, but a last resort is just that...a last resort. In that situation there is no way I turn and run, hoping that both BGs have worse hips and knees than I do, but drawing the gun when another weapon is already in play seems prudent. If in the same situation and two imposing guys approach with no weapon shown, IMO that's the time to indicate you have a weapon but not produce it... that scenario evolves into drawing the gun if the BGs keep closing, two on one (threat). I'm not shooting the guy at 15 feet just because he has a knife, but if he closes to ten knowing I have a gun on him we've reached last resort. The OP was in a parking lot late, where presumably there were decent sight lines and distances to work with, including backing while drawn, and hopefully getting something between you.... this notion that if it's time to draw it's time to fire, means waiting until the knife is danger close. No thank you. Firing before the knife is danger close is going to invite all sorts of second guessing.
I fear a more real world situation would involve more of an ambush than being walked up on.