Final answer:
To determine the best vertical spacing between lines in a brochure, the designer should measure the leading.
Step-by-step explanation:
When formatting a brochure, the designer should measure the leading to determine the best vertical spacing between lines. Leading refers to the vertical space between lines of text in a document. By adjusting the leading, the designer can make the text easier to read by providing more or less space between lines. For example, increasing the leading can help improve readability for longer paragraphs or sections of text.