This is how I do a quick check and keep the hands clean.
Put it on BIG:BIG and see how much "slack" you can pull.
Make sure you have "some".
![](https://cimg0.ibsrv.net/gimg/bikeforums.net-vbulletin/444x356/chain_length_d1b780fbd48c4074b42163d65ee4777b6d7e1b47.jpg)
IF you can't pull "enough", don't shorten.
It is what it is and you really don't want those extreme combinations anyway.