Implementing Weighted KPIs in SSAS

I have a case where the KPI target for each product in terms of sold quantity is different.

So for example the MinTargetQTY for product1 is 100 and the for product2 is 20.

Now when calculating the KPI for the whole store, we need to take into account the overall weighted KPI based on quantity sold for each product.

So, if for example in Store1, 200 items for product1 were sold and 10 items for product2 were sold, then the overall MinKPITarget =

(  ((Product1 KPI * Product1 QTY)+(Product2 KPI * Product2 QTY)) / (Overall QTY) )

= ((100 * 200)  +  (20 * 10))   / (210) = 96.19

and this is MinKPITarget for store 1.

In this case store1 is 210-96.19 =  113.81 items above MinKPITarget (i.e.  around 118%)

Does any body know how to implement this in SSAS...

Thanks

Luai7