I am not exactly sure what you are trying to do, but using a running value function may help:
http://technet.microsoft.com/en-us/library/dd255229.aspx
If you are just getting the percentage of the column or row total, then you can use an aggregrate function like SUM to get the percent. The expression would look something similar to:
=SUM(Fields!ID.Value)/SUM(Fields!ID.Value,"group_name")