The first thing to consider whilst deciding to use a leader is,Is it absolutely necessary. If you are not fishing at long range, upping the diameter of your mainline should be deemed a first option. If fishing in dense weed, a braid mainline is a well worthy and possibly the best option. This is due to its ' cutting through the weed capabilities ' , direct pull, bite indication etc.
I sometimes use 20lb Xline as a leader of about 6ft to 8ft . This is when using a helicopter rig in light weed in gin clear water. Obvious reasons being that it sinks like a brick and hides better than my mainline, and it's strong enough to take the strain of pull on the swivel whilst playing a fish. Sometimes I use a 3in leadcore leader so that the swivel can slide down and position itself onto that. When using an helicopter rig in weed, I use 3lb line to tie my lead on and korda no trace beads as fish safety is of paramount.
When respooling flourocarbon mainline, ensure that it goes on without any twist. It then needs a good good stretch . I go to the local fields early in the morning to avoid dog walkers. I take a rod, reel and my spools along with a Bivvy peg. I walk it out 100 yards or so , peg it down ,walk back and give it a big stretch. I the release it from the peg , walk back and wind in the limp line. I do this 3 times per spool. Hence the early morning lol. It's worth doing this excercise after each fishing trip for at least the first 3 times of use. Or when ever it starts misbehaving. I do it with mono mainline also as I do suffer from line twist from time to time. This excercise resets your line so to speak.