To see how, click Developer > Restrict Editing > Stop Protection.
Then click on the formfield and click Developer > Properties. You'll see that the fomrfield has a bookmark name and an option 'calculate on exit' that is checked.
Then press Alt-F9 to expose the field code for the forumula field.
That's all there is to it.
Do note that, with such a document, you can only enter data in the formfields. If you want to be able to enter data elsewhere, you need to use Section breaks to differentiate the Sections that will/will not have the formfield protection. See attached.
For an idea of what can be achieved with formula field coding, check out my Microsoft Word Field Maths Tutorial, at:
http://windowssecrets.com/forums/sho...l=1#post320143
or
http://www.gmayor.com/downloads.htm#Third_party