I've cleaning and found this in an explanation of finding the SBR of a subject. This was in an old Xeroxed article that was missing many pages, so I have no idea where it came from anymore.

Light - Dark+5=SBR

That would make one photo I took a couple of weeks ago have an SBR of 13

Why would you add five to your difference. The explanation must have been on the missing pages.

I just meter the highest and the lowest and figure the difference to get my SBR. have I been doing it wrong?