Calculate % of custom value in pivot table

Hello all!

I would greatly appreciate some input on the following:

I have a pivot table that successfully organizes my data into one column with ID and one column that Counts the number of Sales made. It looks like:

ID ........ Sales
Susan.....3
Bob........6
Mark......7

I would like to add a column next to sales that calculates the sales as a % of 25. In other words, 25 is my base value. So, it would look like:
Susan .....3......12%

I understand the idea of adding a Calculated Field; however, cannot seem to figure out how to base it off of 25, which is not represented in the dataset.

Happy to clarify further if need be.