How to Get the Direction of an FVector2D

How do I get the direction of an FVector2D? If it is the arctangent, what horizontal would the vector be compared to? I am trying to get the direction a vector points in, the vector in question being the mouse’s location relative to the center of the screen.

Take the screen center vector v1 and mouse vector v2 and then calculate the vector between the screen center and the mouse courser like this v3 = v2 - v1. Now you use v3 and can calculate the angle with a reference vector. If you want up to be 0° it should be the vector (0,-1) i think. Then take the acos of the normalized vector dot product to find the angle. Something like this (have not tested so hope it is correct):

FMath::Acos(FVector2d::DotProduct(v3.GetSafeNormal(), FVector2d(0,-1)));