Tabbing out of any formfield with the 'calculate on exit' set will cause all the calculations to update. That's far simpler and more reliable than trying to have the users do their own (possibly errant) calculations. You might consider making the last field on the document a checkbox to be checked to say all data have been input. It wouldn't matter whether the user exits that field...

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]