Originally Posted by Charles Kenyon

Using legacy formfields in a protected form, if you check the box to "calculate on exit" it will update anything depending on the contents of that formfield.

Here is a link to other links about forms:
Making Forms with Microsoft Word

You may want to use Greg Maxey's add-in when constructing forms using legacy controls. Among other things, it gives you back the lock button.
Add Classic FormField Controls to Ribbon