Help Calculating % Change KPI Indicator

Hi all, I'm trying to create a KPI arrow to show percent change from previous week. I have this which works if I add the Start of Week Dimension but I need it to work without adding any dimensions to the row/column shelves. All I'm interested in is calculating the difference from the latest week in the data from the previous week; I don't care about any weeks other than those 2.

This is what I have that needs to be adjusted without adding a dimension to rows or columns: SUM(Percent) - LOOKUP(SUM(Percent),-1)

Any thoughts?