Use one piece as near to the top as you can which would be just below the strut.
Mount garage door opener 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.
I mounted one off center for a door on my own garage that was 16 wide and it worked fine.
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.
The rail ended up being about a foot and a half off center.
As you said run perforated angle iron from the center stile to the next stile over.
My garage door opener came with the following instructions.