Add rows together depending on another row/column value

Bagpuss 
I think I can get a bit of your requirement but not very clear. You could take get the sum with:

Measure = SUM([actuals to period]) 

Then use slicer to select the value. See example:

If not clear, you might want to look at this post see how to post quesiton with sample. 

Read this post to describe your sample:
https://community.powerbi.com/t5/Community-Blog/How-to-Get-Your-Question-Answered-Quickly/ba-p/38490 

Basically, provide an expect output and clearly describe the logic of the calculation. So we can give a exact dax formula.

Paul Zheng _ Community Support Team
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