I've made a couple of 20" M-14 shorties. In fact, I've tried 20", 19", 18 3/4", and 18.5". The improvement in handling with a 20" over a stock 22" barrel with the flash hider removed, seems to me not to be worth the effort.
18.5" is of course the legal minimum, but with the chance of the bore liner chrome flaking, you can get a bit less than you originally aimed for. So now, I use 18 3/4" for my initial cut, and crown in a teensy bit from there. This works well for me with or without a flash hider.
Look at the pics in the Battle Rifle sticky about folding stocks - it also has info on cutting the barrel.
PS: who care what it looks like ...
the balance and speed of handling, with a GI stock or with a folder, is SO much better with a 18 3/4" barrel.