If you don't feel comfortable with the guns of your F4F, you could try to change the gun convergence. Experiment a little and find out what works the best for you.
You said that you were very close to the Zero. In that case the gun convergence was too far I think. The bullets don't come to one point before they hit the Zero, so they spread on the plane. But don't set your convergence too close or shooting from a distance becomes useless. And with wing mounted guns convergence is always an issue. Shooting with a Bf 109 is a lot easier
