[Adapting effect to changed selection]

I am trying to move from Lightroom / Photoshop to Affinity... so am a struggling with the way of doing things and would be very grateful if someone pointed me (even in broad terms) to the best way to achieve the result I want

In my sample image I want to select the lower part of the picture and change the exposure and other parameters... So I selected the area and refined it... (all good!) 

What I want to do is: Mask this area and apply the functionality... but looking at the result I wanted to **change** the selection (feather it, smooth etc) so that some areas don't show a hard transition (and some areas are not properly selected)

How do I achieve this?

I want the changes in the functionality level eg exposure to automatically extend to the new selected area (or masked/unmasked) area

I did try to paint (white\/black) but didn't get the result

 

Many thanks

 

(attached an example image with selection... and eg the area on the left I want to "correct" after having applied an exposure correction