I have a question about Calculate field in Model builder. My case is the following:
I want the user to choose a field that will be calculated and then I want them to choose another field that will be used to calculate said field. It is kinda hard to explain so I have attached a screenshot.
Here's an older blog post that may be of help. I suspect there are some minor changes in the last several years that may need some adjusting, but at least it will provide some ideas.
Also, I'm not sure if this would be of use, but I'll toss it out there as conceptually it matches a possible solution:
ArcGIS Help (10.2, 10.2.1, and 10.2.2) - Using Feedback in Iteration
Chris Donohue, GISP