Ok lets say you spot meter your shadow and get an EV value of 15, then you meter the highlight and you get an EV value of 18, that is a 3 zone spread. Under the Zone system you would want to give it a N+2 development to get the 5 zone spread for printing....so your N value in the formula would be 2.
Originally Posted by mark
So if you plug the numbers:
If on the other hand you are taking an incident meter and your shadow is 15 and the highlight 18 then yes, the SBR would be 8.
So you see, if you are getting both the spot metering and the incident metering to give you equal readings, then there is something wrong as per your example the SBR by the formula and the SBR by incident metering do not match. Either your incident metering placement is wrong, or one of the functions of the metering is off. When you incident meter the highlights, make sure you are pointing the meter to the light source. I am getting the feeling your placement of the meter is what is the problem.
BTW, you should really ask this question at the www.btzs.org forum, Phil Davis answers these kind of questions personally, so there is nobody more qualified to help you than him.