Create a measure with value from another table

I need to create a measure which is the % of a sales goal for a salesperson.  The total sales for a person is a Measure in a transaction table - TotalTYDSales.  The sales goal, YTDPlan,  is in a separate table with no direct relationship with the transaction table.  The relationship is through the customer table.  I created a Measure in the transaction table called TotalYTDSales.  When I attempt to create a measue of PctYTDSales using the formula trantable[TotalYTDSales] / Plantable[YTDPlan], I get an error:  "A single value for a column 'YTDPlan in table 'PlanTable' cannot be determined.  This can happen when a measure formula refers to a column that contains many values without specifying an aggregation...."

How can I create this Measure?