Thank you Shuhei and Hari,
Shuhei, yes, this is what I was looking for!
This is a first time I am actually working with Table calculations.
Can you help me how to set it up, to learn the process.
I had seen before several, I just spent most of this morning brushing up with Table calc videos. But still can't apply it to this scenario the way you did.
If you learn Table Calculation I recommend to see and understand Tableau's order of operations.
To get 12 months total, you should NOT use dimension filter. Because Tableau applies dimension filter before measure calculation (SUM, AVG and so on).
When you use dimension filter you CANNOT use data filtered out in the sheet.
While, when you use table calculation in filter you can calculate measures with full data, and after that you can hide data according to conditions of table calculation filter.
So I create "Selected Month" filed with using table calculation.
The reason I use LOOKUP function is that I need specify a month in this case.
Argument "offset: 0" means it show itself. ( if offset: -1 it shows previous month and if offset: +1 it shows next month.)
Great! Thank you so much for your help! It is making more sense every time!