Calculating % relative to Total with Multiple Filters in Different Tables

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Hello, I need to use the total from one chart in another chart to calculate the % of each data point relative to this grand total. All data is from the same table. I also need to apply a filter to display only a subset of the values.
When I select a single month, I can achieve this, but when I select multiple months, the data is incorrect.

What's the best way to handle this situation?

Note that I have Looker Studio and don't have access to LookML.

Thanks for your help 🙂