How to set up time period for 12 week rolling percentage

In the "Workforce Management - Workforce Trend Real Time" subject area, in the "Workforce Trend" folder, there are the following fields:

I need to use these in a calculation to determine the termination headcount/average period headcount for the 12 weeks prior to a selected date. I have tried filtering the results on the "Time"."Date" field as shown below:

("Time"."Date" BETWEEN TIMESTAMPADD(SQL_TSI_WEEK, -12, @{SELECTED_DATE}) AND @{SELECTED_DATE}))

This is not working. Is there a way to get the average period headcount for only the past 12 weeks? I know it is possible to do it for Year, Quarter, and Month.