Think you are after a WINDOW_SUM. Attached is your workbook with what I'm talking about.
WINDOW_ calcs are what's known as table calculations. They allow you to show a "roll-up" of both the capacity and also if you wish, the forecasts. (From what you said, it didn't seem like you were asking for that.)
Table calculations depend on the way the view is constructed to calculate. It's like having a way to get the total roll-up in each "pane", "cell" or other pattern when it may not be not available in the underlying data. (imo, the best way to start is to look at your data in a cross tab to try to figure out what you want it to show.)
I created a calculated field (in both of your data sources) with a WINDOW_SUM of the measures you were using. Then, after dragging the field onto your view, you can choose the addressing and partitioning rules, (right click the measure pill, choose "Edit Table Calculation" and then select "Advanced" in the Compute Using: option.
The simple rule I ran across was to "ignore" fields in the view that remain on the left panel (Available fields:) and move everything that matters for the aggregation to the right panel (Compute Using:).
I apologize, but I am just not very good at explaining what's going on behind the scenes. Table calculations are among the most challenging (and powerful) tools Tableau offers.
Jonathan Drummey put together a wonderful blog resource all about this topic:
If you want the formal Tableau explanation, this is a good place to start
Hope this is helpful!
Capacity Test.twbx.zip 230.9 KB