Average Percentage in Matrix Subtotal

I have a matrix like the following:

Period 1 2 3 4 etc. Should be:
name1 3% 1% 2% 3% 2.25%
name2 4% 2% 1% 5% 3%
etc.
Should be: 3.5% 1.5% 1.5% 4%

I want to get the average percentage along the bottom row and the last column.
Using subtotals it just adds all the percentages. Is there any way to do
this? I've seen a lot about InScope but I'm not sure if it would help or how
to use it

These threads might help answer your questions:

http://forums.microsoft.com/MSDN/ShowPost.aspx?PostID=22493&SiteID=1

http://forums.microsoft.com/MSDN/ShowPost.aspx?PostID=648316&SiteID=1