You could utilize a package such as R to do forecasting and control the sum values there but Tableau does not have an ability to do calculations with forecasts. You can vote for this idea here: https://community.tableau.com/ideas/5543
I hope this helps.
The trick here is to 'snapshot' the forecast data - you can do this by selecting Worksheet > Export > Data
When you load this extract back into Tableau you will see the forecast is now hard coded into your dataset. You can then use this in future/subsequent calculations