Any markings will only work at specific speeds, angles, and altitudes. It's like your gunsight, it's a reference, there's no guarantee you hit where it's pointing.
You will improve rocket and bombing attacks by practicing consistent dive angles, speeds, and release altitudes. You can do this offline or in the TA with the green cross showing the impact point. This will give you the correct sight picture, with or without reference marks on the gunsight, and let you practice consistent attack profiles.