I agree with Joe. I also use a 4X (6lb.) tippet but then again, I'm usually not specifically trying for LMB or I would use a heavier tippet. I would use 8lb. test as a bare minimum for LMB but prefer 10lb. test or better. I got lucky when I caught my 10lb. LMB years ago using an 8lb. test tippet. Lucky that the leader held. I like to use the Duncan Loop Knot (Uni-Knot) that allows more fly action but tightens quickly like a noose with tension even on my 6lb. test fluorocarbon. I almost always take five turns when tying that knot. It has been rare when it fails and if it does fail, it usually isn't the knot but the single line that breaks right on the hook eye (this may happen when I don't occasionally retie the knot from continued wear). Normal for a loose moving noose knot (I could leave it tight to the eye and not worry as much about wear but would have less fly action). Same thing for my tippet. I need to constantly check the tippet for wear. Not taking the time for routine leader and knot checks can and does cause me to loose a fish occasionally.