Aim Offset Pitch for Top down game

Hallo guys i have Simple setup of top down/ 60 deg camera angle no full 90
I managed to make aim on mouse direction and Shoot under mouse location (can see in picture)

I would like to know if anyone can show me some example how to make the aim offset pitch to work for this type of game . I tried to modify my existing pitch function but i cant seem to understand how to set it up properly so that the pitch would work -90 , 90 accordingly to my Mouse location in world space ( can see in picture). So that when i aim next to my char pistol is pointing down, when i aim far in distance i point pistol middle and when i aim on some building or high place it is pointing up.

Any help would mean to me a lot
Thnanks