Hi Mike,
Thank you for the details and sharing the query!
It looks like we should be able to view the variable you are creating from the results tab of the MySQL query.
Are you able to run the query and see in the output, under 'data' the @TTax variable?
From the Success query event at the bottom of the query, we can run a separate Javascript Query that console logs the variable from TaxFillTableInv.data to get @TTax to show up in the console!
The JS Query will have access to the 'TaxFillTableInv' and you can key into the .data property with something like TaxFillTableInv.data['@TTax']
I was poking around the forums and found this post 
you will want to do pretty much the same thing but instead of the Action being 'set to local storage' you would select 'Run Script' and console.log(TaxFillTableInv.data['@TTax']).
Let me know if that works!
I just tested it out, and the editor has a yellow line under the query name, on hover it says 'that query is undefined' but this is a false bug and it should work fine. As the query result and its data are created when the query runs!