Numbers Tell a Story

The key is to pick a method of measurement, document it and adhere to it in the future so that when you do another analysis, you have apples-to-apples data.

And how should you treat non-selling space? If white space is brand-enhancing, then it should be considered so for all items in the catalog, not just those on the page where it's used. If non-selling space is presenting a message particular to an identified group of products (e.g., a lead-in spread on laptop computers for a Hewlett-Packard business solutions catalog), that non-selling space could be allocated specifically and equally to the products in the category of interest.