The garage door manual isn't clear about exactly how to check the chain tension. The tension differs if the manual "override" is engaged or not. When it's in "manual mode" (manually open/close the door), the chain sags too much when the door is open, but it's fine when the door is closed. However, if the door is connected to the motor (the motor opens/closes the door), the chain sags too much if it's open or closed (sags a lot more when the door is closed). This seems to match the manual better (it mentions the chain will sag a bit when the door is closed), so I think tension needs to be checked with the motor engaged, but I'm not sure. I don't want to over tighten the chain.
So, do you check tension in "manual mode" or not? If in "manual mode", should the door be open or closed?
The opener is a Sears Craftsman, which appears to be the same one by Chamberlain (the manual looks identical). The website that had the Chamberlain manual was Liftmaster.com.