Percent of sales problem

Dear Friends,

I have a scenario.

I have two charaterstics and one key figure

Year<>Customer<>SalesValue<>%share of sales value

2008<>10257421<>200<>20%

2008<> 1025777<>800<>80%

2007<>10257421<>700<>70%

2007<> 1025777<>300<>30%

here i am not calculaing % share on total sales value

here i am calculating %share on Year level or we can say on subtotal level.

Can anybody help me how to achive %Share of sales value column.

regards,

Malik