Checking garage door chain tension
I found virtually identical manuals for openers from Chamberlain &amp; Liftmaster, so I called Chamberlain support.  She said the procedure is:

Have the door closed.
Disconnect the trolley (that's the thing I was calling &quot;manual override&quot;).  Don't move the door.
Check the tension (1/4 inch above bottom of rail half way along the rail).