One idea I had was not to try to time those fuses to coincide with the incoming fighters' trajectory at all, but rather to create a wall of exploding flak at one distance, at one area in the sky, that the fighter could not avoid. Stagger the shots' timing a bit so he'd be bracketed--he'd fly right into it.
I don't know if I've given a clear-enough description of this idea, but I'm wondering if it was ever tried. Of course, maybe this method was S.O.P. and I'm just displaying my ignorance, which I'm certainly no stranger to.
But there's a whole lot of sky up there, and anyone's guns have ammunition limitations.
I've seen the pictures of the amount of flak in the sky over Germany, and it's astounding that anyone could encounter that and survive. But more often than not, they did, it would seem.