What is the suggested way of defining display-size-independent snap areas?

I want to define a new snap area for the middle two-thirds. How can I define such an area to work on displays with different sizes, resolutions, etc.?

I would like the trigger to be exactly like the one-third options defined in BTT: can be activated with a keyboard shortcut, and does not have any window snapping area that activates it.

Can we see how the middle one-third or similar snapping areas are defined in BTT? This should simplify defining snap areas with arbitrary sizes, including the middle two-thirds.

The "Custom Move / Resize Window" option is a flexible solution that can address this question effectively.