Upon dive bombing, I usually aim with the bottom of the targeting sights. Carpet bombing, I have no clue.
EDIT: Found that not using ironsights and going and a 45 degree angle or further downward will allow you to accurately place your bombs. In real life also, the Germans used the bombsights in the bottom of the plane to spot (Use your look around key in the plane and look downward to see it) and once they spotted something, being some 10,000 feet in the air, they would swerve around going downward, diving towards the target, and then release bombs, then flying away.