I do get a bit confused with the way that you use the SBR terminology Sandy. Coirrect my thinking: SBR is BTZS Incident system terminology. An SBR of 5 is a flatly lit plane without any lighting variation. I get that impression that when you say SBR 5 that you are referring to 5 stops of difference in lighting ratios between lit and shadowed areas instead of no difference. I am no stickler for terminology but if each of us means something different when using the same term the communications of concepts and ideas can get quite muddled.
I also believe that SBR is a term that applies only to a scene.