how to have subtotals of each page in RDLC ?

hi all.
i've read many posts about it. but realy didn't find a logical or static solution.

Q:

i have a object datasource bound to rdlc.
i wanna subtotals of each page to be shown at the buttom of each pages' table,
and have summation of all previous pages (in fact : subtotal of previous page) on the top of each page's table.
i don't wanna limit a group to specified number of rows,
cause my printing target paper is not specified and static . (whatever user wants: A5 , A4 , ...) ,
then it can be 20 rows per page, 30 rows per page or more. so what should i do?
So :
- how to have subtotals of each page?
- subtotal of previous page at the top?

Now, could anyone please tell me what should i do now?
thanks a lot.