dynamically determine a row or textbox

I have a report using a list and a table. The number of rows in the details
section of the table is determined by the number of records returned from
the query and the report automatically adds detail rows dynamically . My
problem is I have a page footer and I want to put in a calculation to
compute a percentage. The calculation will divide the value of the textbox
in a cell in a column in the last row of the detail by the value of the first cell in the column.

If I do not know the number of rows in the detail section and the textbox
name at any given time, is there a way to programically determie the # of
rows in the detail and the texbox names ?