Revise the sentence that begins with "Use the following steps..." and step 5, and also add a paragraph to the end of this section:
Use the following steps to determine the percentile of rainfall events:

5. The USGBC calculator determines the various percentile rainfall amounts. If using another spreadsheet software, apply a percentile function (or similar) to obtain results.

Detailed explanation regarding calculation of the percentile event using daily rainfall records can be found in EPA 841-B-09-001, December 2009, www.epa.gov/owow/nps/lid/section438.