Baseline Sales

Sam_Jarvis , Make sure you have date/week table with YearWeek (YYYYWW) or week start date 

Create a rank column on that

Week Rank = RANKX('Date','Date'[Week Start date],,ASC,Dense)
OR
Week Rank = RANKX('Date','Date'[Year Week],,ASC,Dense) //YYYYWW format

You can have measures like

These measures can help
This Week = CALCULATE(sum('Table'[Qty]), FILTER(ALL('Date'),'Date'[Week Rank]=max('Date'[Week Rank])))
Last Week = CALCULATE(sum('Table'[Qty]), FILTER(ALL('Date'),'Date'[Week Rank]=max('Date'[Week Rank])-1))
Last year Week= CALCULATE(sum('Table'[Qty]), FILTER(ALL('Date'),'Date'[Week Rank]=(max('Date'[Week Rank]) -52)))
Last 8 weeks = CALCULATE(sum('Table'[Qty]), FILTER(ALL('Date'),'Date'[Week Rank]>=max('Date'[Week Rank])-8 && 'Date'[Week Rank]<=max('Date'[Week Rank])))
last two weeks = CALCULATE(sum('Table'[Qty]), FILTER(ALL('Date'),'Date'[Week Rank]<=max('Date'[Week Rank])-1
&& 'Date'[Week Rank]>=max('Date'[Week Rank])-3))

Power BI — Week on Week and WTD
https://medium.com/@amitchandak.1978/power-bi-wtd-questions-time-intelligence-4-5-98c30fab69d3
https://community.powerbi.com/t5/Community-Blog/Week-Is-Not-So-Weak-WTD-Last-WTD-and-This-Week-vs-Last-Week/ba-p/1051123
https://www.youtube.com/watch?v=pnAesWxYgJ8

Thank you, I will give this a go.

Do you have any advice on creating a baseline average, removing promotional weeks.  I assume there would need to be a calculation to determine promo

weeks - e.g. ASP$ change more than x, then remove those weeks to determine average baseline?