Andrew,
Sure. If you are using a protected form, you can use expressions in
calculation type formfield. Insert the fiedl, double click and change type
to calculation. You must also set each variable field to calculate on exit
(double click each field and select calculate on exit).
If you are using a non-protected document as a form then you can simply add
formula fields. Type "formula field" in Word help for tips to get you
started.