Reference Textbox In Other Textbox

Hi!
I dont know if i will explain this correctly, but my problem is with reporting service.
I'm supposed to Sum value in one textbox and than that sum use it in sum in other textbox.

Something like this:
Sum(Fields!Abc.Value/(Fields!dfg.Value+Sum(Fields!abc.Value)),"matrix1_RowGroup1")*100
I get error msg 'The Value expression for the textbox 'textbox49' contains an aggregate function (or RunningValue or RowNumber functions) in the argument to another aggregate function (or RunningValue). Aggregate functions cannot be nested inside other aggregate functions.'

Something like this wont work either Sum(Fields!abc.Value/ReportItems("textbox56").Value)*100
Error The Value expression for the textbox 'textbox55' uses an aggregate function on a report item. Aggregate functions can be used only on report items contained in page headers and footers.

So, pls help if you know how to reference textbox in other in body of report.
Thx.