Hello fellow Legacy builders who are farther along than I am.  

I'm installing the fresh air vent closeout panels.  The construction manual
says "Trim the opening to the scribe line after bonding."

Well, I don't have any scribe lines on my closeouts.  How does one determine
where (how far back from the front of the opening) to place the vertical trim
line?

Thanks in advance for the help!

John Kleber