Adding values in formfields

I am trying to create a function that will add the values of 2 text boxes
based on if the checkbox is true or false. I am using Word 2003 but it needs
to work on Word 2000 also.
If the checkbox is true then I want the value of Copy divided by Pages
If False then I want the value of Copy times the value of Sheets.

Currently: Copy, Pages, and Total are all textbox form fields
If CheckBox1.Value = True Then
ActiveDocument.FormFields("Total").Result = ActiveDocument.FormFields
("Copy" / "Pages")
Else
ActiveDocument.FormFields("Total").Result =
ActiveDocument.FormFields("Copy" * "sheets")
Endif

Can this be done somehow? Would I have to use something other than
textboxes? The users needs to be able to update the formfields with their
data. Thanks for looking.

Josh