Amusing relationships abound everywhere. Doesn't make them important. To me this is just like 3 out of every 4 people in the world make up 75 percent of the population. Or that 100 percent of those born will likely die.
My plumb bob hangs three or so inches in front of the spindle when the pedal is forward and horizontal to the ground. Not certain though where KOP's comes from. The front of the knee? middle?
Don't know, don't care. It's just a mythical relationship that I admit can be used to get a person in a starting point to begin adjusting various things. However when people say that you must stay in KOP's over all else. Or, refuse to try a position because it takes you outside KOP's, then I have a gripe.
Balance depends on a lot of things. And balance where? On the seat, while not moving and in a fairly upright position? No, that's not me. Balance is a dynamic changing thing from power you are putting out, your airspeed (not ground speed) as you move, your bodies core strength and its natural tendency to only bend so far before it leverages some of your weight out of the seat.
For me I can only do that with a saddle forward and a fairly aero position. Balance is more my power balance around the BB, not on the seat. With my saddle back, I have to have a more upright position to be comfortable. And that means using more watts on long rides that I don't want to give away. If I was more disposed to leisurely riding, then maybe KOPs would work. But only because it's a coincidence, not a must do this above anything else.