The question seems much more difficult than intended when asked. There is a really big difference in incident and reflected meters (camera and spot meters are reflective). They are used very differently.
Incident meters are aimed at the camera (away from the subject) so that they directly meter the actual light value ON and AT the subject. By metering the actual light intensity (ignoring the actual subject and its colors), this makes shadows appear as shadows and highlights appear as highlights. Get the light right, and it comes out right.
Reflective meters are aimed at the subject, and see only the light reflected from the subjects colors. Different colors reflect differently. which fools reflected meters.
A black dress reflects little light, and reads low, and the meter adjusts to make it come out middle gray tone, i.e., overexposed.
A white dress reflects a lot of light, and reads high, and the meter adjusts to make it come out middle gray, i.e. underexposed. And white background walls also tend to cause underexposure too.
So neither can be assumed correct exposure, a human brain has to interpret it, and compensate the meter reading accordingly.
A Spot meter is the same, in that it simply makes the selected spot come out middle gray.
Whatever you aim a reflected meter at will come out averaging middle gray level (not necessarily gray, could be colored, but meaning a middle tone). Which of course is not necessarily accurate, unless the spot was selected as one that SHOULD be middle gray.
The spot meter can isolate the face from the surroundings, but we better know that we have to apply maybe +1 EV compensation if we don't want the light face to be middle gray tone.
That is simply how reflective meters work, only way they can work. We have to learn how to compensate for the reflectivity of colors for the reflective meter. This is pretty much automatic for the incident meter, independent of subject, so no issue there.
See http://www.scantips.com/lights/metering.html
