For trad climbing, I normally carry shoulder-length slings racked on my harness, set up in Alpine draws at 1/3 their full length. When I'm leading and need to extend my protection, I either use them at this length or extend them to their full length.
There are some cases, though, where I find that I want 1/2 the length, not 1/3, and I want something I can securely girth-hitch. Specifically, when I'm extending my ATC for rappelling, 1/2 the length is just right, while 1/3 is too short and doesn't keep my autoblock from getting into my ATC.
What is the best way to shorten a sling to 1/2 length for this purpose? Let's say I want to hitch to my belay loop and connect to a biner on the other end.
The simplest thing is to put the sling through the belay loop, which then gives me the two ends of the sling hanging out of either side. If I bring these ends together, I can clip in with a biner. For reasons explained below, I think this is a very poor method.
I can tie a butterfly in the sling and adjust it to get the desired length.
I can tie an overhand or figure-8 in the middle of the sling and load it at the knot.
Method 1 is lousy, because the strands aren't redundant, and furthermore when I'm taking it apart there is an extreme tendency to drop the sling, which isn't securely girth-hitched to anything. I could fix these problems by tying an overhand in the middle, but that makes the sling much shorter.
Method 3 is what I used to do, but I've seen this criticized because the knot is sitting right on the biner. However, if the knot slips, the system doesn't fail, it just extends.
Method 2 is what I'm doing right now. It seems OK, although it's difficult to properly dress the butterfly when you make it in a dyneema sling, and it can be a little time-consuming to untie. It requires some fiddling to get the bar tacks out of the way before you tie the knot. This method uses the butterfy for its intended purpose and loads it in the intended way. If the butterfly pops undone, you get extension but not failure. An advantage is that you can adjust the length quite a bit.