The hemisphere dome should be out for ALL BUT the shooting of flat objects (e.g. paintings).
The retracted dome should be used for
- the shooting of flat objects (e.g. paintings), or for
- taking readings of individual light sources to calculate ration of light emitted on the subject by each source
You want to measure the light striking the face, so you want to be somewhat close to the face so as to measure light as it strikes the face...if you are too far in front, you might not be in the 'same light' as the subject.
You should point the hemisphere AT THE LENS, to measure the light as seen by the camera. You point a meter at the SOURCE, only to measure relative intensity of individual sources, but NOT to measure what f/stop to set on the lens for exposure.
(There is a school of metering that says to point the hemisphere AT THE MAIN -- but I believe that technique is to minimize the possibility of having highlights overexposed and blown out on transparency film; and pointing meter at the lens prevents muddy color on color neg....pointing at the lens also is of benefit then in inherently practicing ETTR for digital to some degree!)