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Jorge --
Calculations involving N-numbers require the "zone spread," which is the difference between the chosen zones. For example, if you've used zones II and VII the spread is 5 (not six, as Minor White decided). Then, if S= the SBR in stops, D= the spread (zone Difference), and N= the N-number, then S = (7*(D-N))/D
You'll find this and other (sometimes) useful equations in the appendix of BTZS 4th. Don't trust earlier editions; they somehow got some of these things thoroughly screwed up. Also see fig.9-7, page 133 for a chart that may help to explain these relationships. If you run the Plotter N/Dev chart several times with different spread values you'll also see why BTZS doesn't use N-numbers at all.
Caution: not all zonies calculate N-numbers the same way so this equation may not work for everybody.
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