This might seem to be a simple approach, but for some it takes a while to figure out that such tricks are even possible in SSRS.
For example to calculate the difference/variance between Actual and Budget values in a report:
=ReportItems!Textbox50.Value - ReportItems!Textbox171.Value
Simply lookup the textbox’ name in the Properties-window:
This approach also works for ReportItems outside of the table/matrix (across the report even).
In some cases you’ll need to combine two KPI values from different matrixes/tables in the report and for example run another formula to come up with a definite rating/value.
Instead of ‘Fields!‘ use ‘ReportItems!‘ to refer to actual items on the report canvas (this can be on the header, footer or body of the report).
In my experience you may need to use a temporary hidden textbox to store the value temporary in order to use it in the header or footer. So in case you run into some sort of constraint, try this one out first.