Pressure Drop over a Rectangular Opening

SMACNA and ASHRAE show the C-values. They differ for air entering, or exiting.

Also some here

Thanks, couldn't find much in ASHRAE but SMACNA had a lot of C-values. Nothing for a wall openings but I chose the closest I could get from their duct coefficients.

In most cases it flows from a duct to a room. but it also makes a difference if the duct discharge is in the same plane with the wall, or if the duct sticks out.
If you find multiple values, use the most conservative value.