Braid with Fluoro leader.

[MuskieBait]

The fluoro leader does not need to be too long if your intention for the leader is for...

1) lower visibility

2) abrasion resistance

3) small amount of line to allow stretch

In most cases, a 3-4 feet of fluorocarbon is adequate. I usually use a uni-to-uni knot if the diameter of the line is "close enough". In cases where I'm attaching a heavy fluorocarbon to heavy braid, where the knot becomes a bit large to pass through the line guides, I would use a slim beauty knot (actually, mine is a kind a hybrid knot between a slim beauty and a mid knot). When I say heavy braid to heavy fluoro, I mean 50lb braid to 30lb or heavier fluoro.

For your case with bass fishing, it really depend on where you are fishing. If you are fishing in heavy cover where you may need to drag a fish out of the cover (wood, heavy weed, lilypads...etc), I would use a heavier fluoro leader such as 20lb or heavier. Actually, I would use a heavier braid too in that case. If you are fishing in area without much cover with the 15lb braid you proposed, you can use a lighter fluorocarbon leader such as 10-15lb. However, you have to be aware that 15lb braid has the same diameter as 4lb mono, so the uni-to-uni knot may be more difficult (but not impossible) to tie. You have to make sure the knot is tied properly (the loops and wraps of the knot lies properly once tied, and the knot was lubricated before tightening). But again, if the braid diameter and fluoro leader diameter is hugely different, I would try the slim beauty knot.