Azul, heres my rules when I bomb. When Im dropping on any kind of hangers I use a delay of .05. This is more like precision bombing, works best with larger ordnance. When hitting a start factory or a town I generally use a delay of .10, used with smaller ordnance its a killer.
I believe its the B17 that can carry 20 smaller bombs. Head to a radar factory, worth the most perks, salvo all and make a delay of .10. Then make a diagnol pass on the factory, one corner to another and you will devestate it.
Also, when going for a cargpet bomb effect, if you somehow lost your drones, I usually salvo about 1/2 my load and have a sorter delay. This alows for a second pass.