Replicating a filter in DAX

Hi, I am very simply able to calculate something using the filter pane to alter the context and display a correct number. Please see this image.

However, I now need to run some calculations on the number presented by this, which means I need to write some DAX to obtain it instead of using filters.

I have experimented with TOPN but I can't seem to get hold of the right number.

Has anyone got some ideas on how to exactly replicate this filter in DAX?

Thanks in advance!