Sorry for the delay in receiving a response. The best way to accomplish this would probably be to reshape your data. I've reattached the Excel sheet with the data reshaped (refer to Sheet1-Tableau)
Then, a couple of calculated fields can be created.
total(sum(Value)) --this one should be computed by Store
total(sum(Value)) --this one should be computed by Product Name
Then, created a third calculation that divides the second field by the first.
Hope this helps!