Rolling 6 month average with non-consecutive dates

Hello,

I'm trying to build a 6-month rolling average of Sales $.

However, my sales data is aggregated by Week Starting Date, so the usual formulas don't seem to work. 

I currently have something that just adds up sales $ from the last 6 month numbers, but for every year. I understand why this doesn't work, but it's the closest I've come to a solution. I've also tried modifying the solution presented in this post with no luck https://community.powerbi.com/t5/Desktop/How-to-create-rolling-averages-over-non-consecutive-dates/m-p/1410175#M596833

Any suggestions would be greatly appreciated.

Current formula:

Here is a sample of the data and the result comlumn I want:
I would like each week to be the average of the 6 months/26 weeks before it.