The way Workfront works is that you're able to use a custom calculated field on different forms, with different calculations each time. As a result, every time you pull in a calculated field, it is blank and you must recreate the calculation (either by copy and paste or some other way).
If you are getting the multi-use form error it's because somewhere in your system there is an object that is using two forms, and both these forms have the same calculated field with different calculations.
Your mission is to locate this/these object/s and
option 1: reconcile the calculation so that they are the same for both forms.
option 2: stop using multiple forms / stop using the same calc field in more than one form.