If it's a single variable on a PDI display you will need to make a DataSet. When you add a variable (value or bar) to a PDI display it go to the Define Value dialogue. Click on the drop down next to Tag Search and select PI Calc. Create a new Data Set and then in the PI Tag or Expression field enter the following expression but replace sinusoid with you tag name.
This is how it would look. you can add any calculation (using PI PE syntax) to your "Dataset" and it would get calculated at run-time.
Just double-click on the tag that you added to the display and "Define Value" pop-up would appear. Select "PI Calc" from the drop-down and configure a new "Dataset" as shown or depending on your requirement. You might have to create one data set for each value you want.