To use this you have to use the custom calculation option. See here for more info on the subject:
http://acrobatusers.com/tutorials/how-to-do-not-so-simple-form-calcula tions
To use this you have to use the custom calculation option. See here for more info on the subject:
http://acrobatusers.com/tutorials/how-to-do-not-so-simple-form-calcula tions