You have forged your relationship using the diagram view within PowerPivot. You've created your calculated field. However the data doesn't seem right to you. If you slice the data one way the figures are fine. However when you include fields from other tables the data is inaccurrate.
This training video will help you see how you can use the DAX Calculate function in PowerPivot to create relationships from tables that aren't directly related to each other.
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