Percentage Column in Matrix

Hi,
I would like to generate following table with reporting service matrix.
But if I use the expression : percentage coulmn=sum(field!qty.value) / sum(field!qty.value, "region_group")
It will become a percentage of a row total, instead of a group total (store group). Please see the second table.
Any ideas?

Thank you!

(Correct!)

store1 store2 level1 level2 level3 sub total level1 level2 subtotal category region qty % qty % qty % qty % qty % qty % qty % C1 APAC

10

10%

20

20%

70

70%

100

100%

25

50%

25

50%

50

100%

EURP C2 CHINA

(Wrong)

store1 store2 level1 level2 level3 sub total level1 level2 subtotal category region qty % qty % qty % qty % qty % qty % qty % C1 APAC

10

6%

20

13%

70

46%

100

25

16%

25

16%

50

EURP C2 CHINA

Can somebody help me or give me a hint? (or my question is not clear enough?)
If this kind of report is doable, I will ask my team member change their JSP report to MS RS.
Thank you!!