My take on incident readings is as follows: The white dome is a stand-in for the subject. The meter should be held close to the subject, or in such a way that the light falling on the dome replicates the light falling on the subject when the dome is pointed towards the camera lens. In other words, if the subject is lit with side light so that it is half in shadow, the dome should likewise be half in shadow. Always point the top of the dome towards the lens of the camera.