When working with multiple data sources, the aggregating function you need to use in order to compute totals is WINDOW_SUM(). TOTAL() would not work on calculated fields that reference multiple data sources.
I came up with a hacky solution to your problem. I compute the grand total in a separate sheet, and then combine the 2 sheets in a dashboard.
First, I define the following calculated field, and name it GrandTotal:
I need the IIF so this only shows for the last store. If you just use WINDOW_SUM([PerTran].[TranTotal]), it will show up for all stores.
Create a second sheet, and drag Store to Rows and GrandTotal to Text. Select all stores except the last one, right-click and choose Hide. Now right-click on the last remaining store, and deselect Show Header. You will only get one number - the total - displayed.
Now you can combine the 2 sheets in a dashboard: choose floating layout, drag sheet 1 onto the dashboard, and then sheet 2 immediately below. You will need to play with the layout to get it precisely positioned. On Sheet 2, you can also right-click and deselect Title so that it appears as a single table in your dashboard.
Note that I did not sum the PerTran and Transaction columns, as it doesn't make sense. You only want to sum the product of the two.
Check the workbook I'm attaching, and let me know if you have any questions.
By the way, it looks like this is a problem that has been asked before, but I'm not sure how much the users were satisfied with the example provided as an answer: it does appear that it contains a column that was incorrectly totaled. Please check this out:
GrandTotalMultipleDS.twbx 12.2 KB