Calculating Percentage using different tables

Hi, 

I have two tables, one database of shipped sales (Digital_Sales) and of refunds processed by a distribution centre (Database)

The two are connected via a 3rd and 4th table linking the 'date processed in dc_date' to 'fin year period'. 

What I'm trying to achieve is to get a visualisation of refunds as a % of sales grouped by period like in the excel example below. 

I've tried performing this with the below measure.

Measure = sum(Database[RETURN_VALUE_SUM])/sum('Digital Sales'[Shipped_Sales])*100

I think the problem is the connection (see the bottom). When I build a table for return sum and sales sum by period, it's just summing everything for the sales.

Any help will be greatly appriecated.