An article of note on this topic is:

Sapp, Gregg and George Suttle. A method for measuring collection
expansion rates and shelf space capacities. Journal of Academic
Librarianship 20, no. 4, 1994.

  The input on this thread matches Sapp & Suttle's approach, with the
important difference that they chose to count each 36" shelf as 28" of
usable shelf space capacity. It is important to choose a shelf space
capacity number that gives you some leeway. For instance, you don't
slice a 3" thick bound volume in two to fit on two shelves, so 2" might
remain empty. Many other things can occur to throw off exact
measurement, but counting a shelf as 28" instead of 36" gives you the
wiggle room to accommodate variations.

Steve Black
Reference, Serials, and Instruction Librarian
The College of Saint Rose
392 Western Avenue
Albany, NY 12203-1419
blacks@strose.edu
(518)458-5494