How do I measure this

[Porsibygg]

Both easiest and I get it tight. Sits flush against the floor molding but doesn't go all the way up to the ceiling panel.
I have a hole saw but don't want to be the one who sets the hole crooked.
The board is 60 wide.

 

[Småbrukaren]

Use a template, like cardboard or something similar. Or aim a laser at the center of the valve. Mount the disc and saw where the laser points.

 

[egge80]

Remove the vent, measure the center from the ceiling and the center from the previous panel. Glue up the new panel, mark your measurements and saw. Press the vent in place, take two steps back and enjoy having a centered hole.