Currently Tableau Desktop does not have the ability to use forecasted values in calculations.
The Tableau website has an Ideas section where customers can add, comment, and vote on features they would like to see in the product. Votes are used to help prioritize new features. Our Development team also loves to see everyone's use cases in the comments, which helps them better design features that will meet the most possible needs.
The best workaround I've seen is to export values to Excel or CSV and then use that file as your data source. However, this does add a lot of manual steps to the process.
Hope this helps!