Recommended sling length for autoblock?

As the title says, what length sling would you recommend for an autoblock for a rappel assuming I'm using 6mm accessory cord?  Some of the posts I see say anywhere between 12" to 60", which is a pretty big range! I suspect I'm getting a bit confused because they don't specify if the length they're describing are the pre-tied length of cord or the length of the tied loop itself.  Thanks!

find a way to mock up two anchor points and set yourself up on rappel however you like. take a 60cm sling and set up an autoblock. do as many wraps as you feel like you need. measure any excess material and subtract from the sling length. then go to REI and get that length of cord, plus a few inches extra for the double fisherman's.

14 inches works great. 

I typically do an extended rappel with third hand attached to the belay loop, similar to this. The13.5 inch Sterling hollow block is nice and works perfectly for me in this configuration. 

Of course you can make your own out of cord. You could make the loop around 14 inches and it would be somewhat equivalent to the hollow block, but you may want to make it a little longer to allow some room in the setup for the knot and a possible extra wrap because cord won't grab as well has the hollow block. You'll find that the double fisherman knot might consume more cord than you expect, but it's easy to estimate by tying a knot and then measuring.

If you already have the cord, just make the loop a little longer than you think you need and experiment with it somewhere safe. You can always make it shorter. But if you didn't buy the cord yet, I suggest just getting the hollow block.