Its going to depend on how long of leader you plan on fishing and the type of fishing. But i would say about a 1' or less of butt section, that will allow for your short leaders to be short and your long leaders to be long, but if you go with a long butt section say and you want to fish a 6' leader your either going to have to make your own or just dont fish a short leader, and the same applies on the long leaders unless you want to start fishing 15' leaders. As for the perfection loop, do whatever you can to cut that tag end short, there are only a few knots in the world of fly fishing that require a tag end left long and that is not one of them. If everything is done properly in the set up you shouldnt even notice the perfection loops in there.