According to the SBL Handbook of Style blog, the example given for citing NPNF in the SBL Handbook of Style’s second edition (pg. 101) is inconsistent:
Contra the example given in SBLHS, the series number is best indicated by a 1 or 2 plus a solidus preceding the volume number (not a superscripted 1 or 2). Thus volume 12 of the second series would be cited as follows:
NPNF 2/12:85–96
For additional discussion of SBL style, see SBLHS’s original post, as well as these related posts.