The opener can be installed within 4 feet to the left or right of the door center if there is a torsion spring or center bearing plate in the way of the header bracket or door bracket area.
Mount garage door opener off center.
I mounted one off center for a door on my own garage that was 16 wide and it worked fine.
To offset mount the opener arm to the door section.
The rail ended up being about a foot and a half off center.
I had to do it that way because i had virtually no room over head and the spring was set right in the center not allowing me to center the rail.
The great thing with wall mount garage door openers like the chamberlain rj020 is that they save a lot of space when it comes to ceiling use a common problem with most ceiling mounted garage.
If your door has extension springs the opener must be installed in the center of the door.
Make sure there is a strut on the top section it s required when using an opener run a second piece 6 to 12 below it.