DP- Macro for drill down calculation

Need help with creating a macro for the following :

Planning book/ data view:

Product

Wk1

Wk2

Wk3

Sales Qnty

100

10

30

5

101

102

20

40

20

103

104

Macro should drill down at product level and when ever the product is “100” it should do the following calculation:

Sales of Product ( 100) for Wk1 + Sales of Product (102) for wk1= 30

Find % difference for product 100 for wk1 with the total = (10/30) *100= 33.3

Generate alert if the % difference is NOT between 40 and 50. This should be done in same way for other weeks. Calculation is only required when the product is “100” in drill down.

I can use DRILL_DOWN( 'product' ) for drill down. I not sure what functions can be used for the calculation.