Creating Index Using Grand Total Value

Hello Everyone,

I am trying to create an Index value using the Grand Total value and not able to find a way to do this.

I have a database of actual sales by sku by week. For each week (I can have same week for multiple years), I want to create an index to see seasonality. For that, I created a row calculation showing average/week for each week. Now I want to divide this value by the Grand Total value (which is average of all the weeks that are selected in the table). I am not sure how to do this. I am attaching the work table for reference.

Objective : Create the "Index" field which I can do fairly easily in Excel but can't figure it out in Tableau. I am attaching my Excel workbook.

Any help will be appreciated.