"automatic" AutoSum - can I insert a button?

I am trying to update a performance review form for my office. One section
requires managers to input the percent of time spent on each job duty by the
employee. The departments want the bottom of the column to add up the
percentages so they don't have to figure out if the numbers they've entered
add up to 100%. I am trying to figure out a way to insert a button next to
the FINAL TOTAL cell on the table so the managers can easily see the new
total with one click of the button. We are using Word 2003 at the moment.
Any help???