Elliot I fully agree that where and how the head is oriented for each reading does make a distinct difference to the SBR calculation.

Your reference to BTZS does bring up a good point, when we talk about and try to mentor others our frame of reference is very important.

In my frame of reference and practice, when duplexing or measuring contrast I always point the incident meter directly at the light source for the highlight measurement then I measure parallel to the camera axis. I use these two because the highlights and the mid-tones are of most importance to me personally. Typically I only feather the meters head to bias my exposure when using a single dome out reading.

For others, shadows and mid-tones, or shadows and highlights, may be more important to peg, there's no rule against picking different lines and places to measure, or to feathering the meter head one way or another, those are very reasonable artistic or technical choices.