Let's take a Revit project where you have a casework family loaded. As an example, you want a casework tag to take the Width and Depth parameters, add them together, and display the result as part of a tag label.
I have included one approach below. Please keep in mind this is for component or in-place component families:
1. First edit the family, and add a new shared parameter. This shared parameter will be used to add 2 existing parameter values in the family.
2. For this example, we will simply add the existing Width and Depth parameter values. In the Family Types dialog > Parameters > Add > Shared Parameter > Select, and either choose an existing shared parameter or create a new one. Add the shared parameter to the family.
3. Once loaded into the family, enter the desired formula under the Formulacolumn. Ensure the results appear as expected. Save the family, and load into the project.
4. Next, create or edit your specific tag. In this example, I created a new Family > Annotations > Generic Tag > and changed the Family Category to Casework Tags.
5. Add a label, or edit an existing label. Click Add Parameter > Select > and choose the Shared Parameter you created earlier and entered the formula for. Load the tag into the project.
6. In the project, tag your family with the new tag, and the label should represent the result of the family shared parameter.