Actually Elliot, two things, pegging to and understanding the highlights is quite easy with an incident meter and exposure placement changes do not change the SBR.

When we use an incident meter pointed directly at the light source, we get a reading that protects the highlights nicely. This is a direct objective measurement of the real luminance involved, it can be expressed as a real number that our industrial photographic process equation understands.

To put this into to practical terms, think front lit subject matter mid afternoon on a beautiful sunny day, the perfect "Sunny 16" situation. The utility of this measurement is well proven. Given a tested E.I. is being used, there is no question about whether or not the incident meter pointed at the light source in that situation will provide good technical placement, it will be right period. The only wild card here is if the photographer has calibrated him or herself to the system and any artistic bias.

Life is rarely that simple though. If our subject is in that situation but their face is turned away from the sun say 120 degrees we are faced with a problem. Most of their face is not not lit by the sun directly but some is. Most of the face is now lit by open sky and the reflections off the landscape. In this situation we get a second reading with the meter pointed along that 120 degree line back toward the camera. That reading alone will get us a perfect exposure for the subject as lit by the open sky and reflections.

At this point we have the info we need to decide on how to deal with both light sources. A simple averaging of these two readings is simple duplexing. It is a compromise. It does its best to protect detail from both lighting situations. It allows a little shadow and a little highlight detail to fall outside the straight print range. This is exceptionally useful for slide shooting. It also provides the info needed to make decisions about contrast, maybe we choose Astia or Provia here instead of Velvia to adjust to the SBR we decide we want. With negatives we can decide on development choices because we have the info needed to know if contrast is "normal" for us or not.

Onward, SBR is a chosen range that we pick out of the scene. It isn't effected by anything else.

With negatives as long as we get our whole chosen SBR somewhere between toe and shoulder on film, the exact placement doesn't matter to the contrast calculation. Sure an underexposure where we let things fall off the toe or overexposure where highlights fall off the shoulder are a problem but if we get in the middle there is no loss nor gain of SBR. Anything on film but outside our chosen SBR has to be accessed by using burn or dodge or changes in paper grade.

Our challenge, when our SBR is in the middle, is matching our chosen SBR to our paper.