asp.net Datagrid issues

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C#

hi i'm new at this posting thing so here goes...
I've got a datagrid with some amounts in one row and a checkbox in another. I also have a calculate button. When i click the calculate button, i want the code to go through the datagrid and calculate the amounts of each row that has been checked in the checkbox, if a checkbox is not checked, it should not use that amount. I'm stumped, but here's the coding i've got so far. It's called from the button on_Click event:

foreach (GridViewRow rows in dgServices.Rows)
           {
               CheckBox cb = (CheckBox)rows.FindControl"cbAddTo");
               if (cb.Checked == true)
               {
                   CalculatedItem += float.Parse(dgServices.Rows[0].Cells[8].Text);
               //here's the problem in the .Rows[0]
}
               else
               {
          //do nothing
               }

               txtTB4V.Text = CalculatedItem.ToString();
           }

so basically i need to know if i'm on the right track or if there's an easier way to do this. Thanks in advance :)

[edit]Code block added - OriginalGriff[/edit]