Brad,

Several ways to update fields.

Select it and press F9
CTRL+a (selects all) and press F9
If you have your print options set up to update fields at print, then
you can toggle to and from Print Preview.

If it is just fields in the maintext story of the document you can run
the following macro:

Sub NoName()
ActiveDocument.Fields.Update
End Sub

If you have fields in other text stories you can use:

Sub UpdateAllFiled()
Dim oStory As Range
For Each oStory In ActiveDocument.StoryRanges
Do
oStory.Fields.Update
Set oStory = oStory.NextStoryRange
Loop until oStory is nothing
Next
End Sub

None of these methods are automatic.  If you want the results to be
completely automatic you will have to perform your calculation using a
protected online form.