I use them, and have for years.(made up leaders) I had lots of mono material in manufacturing a leader/lure. Basically it is not that critical that you get things exact given the modern graphite rods. I wouldn't use a formula, and there are many, many formulas, that has a lot of breakdowns...butt material, several diameter changes, then an 8",8" 8", 8" etc. That is not necessary. Mine turnover fine, and I go with about a 1/3 of the leader the butt piece length, and 1/3 the tippet. Actually my tippet is slightly less than that.( 10' leaders) For the butt I start with say 3-4 ft of .021 mono, followed by a 16" piece of .018, then 12" of .015 then 12" of .013 then 10-12" a length that will work with whatever tippet I use for that leader. If it is 1x I will tie the length right to the .013. If it is 4x, or 5x I tie in a 10 to 12" piece of .009, and then the tippet. I use nail knots on the line end, and lately have used a loop having learned how to weld a small loop in the end of my flyline using clear heat shrink, and shrinking it, and welding the flyline coating using a candle. I did this because I bought several furled mono leaders that I only have to tie a tippet to. I got a deal on them last year at our big fly tying Expo from an exhibitor, and realy liked how they turned over, and they are made with the small looped butt. I don't like a big loop in the end of my flyline, but can now take a line that has no loop, and make a small looped end. And here is a new add on that I really like. The weakest connection is the tippet to leader connection usually. Doesn't happen to me on fish, but get hung up in the bushes, and the tippet knot often breaks before the tippet to fly knot. SOOOO, I tried these tippet "O" rings...metal rings that are about the size I typed. They come in different lb tests 15 lbs beign the smallest, and 30 lbs the biggest. I like them. YOu don't have to match up your leader to tippet diameter as exactly. I end the leader at the small "O" ring with 0X, and an then tie on virtually any tippet with a clinch knot. They cause no sinking problems, or visibility problems. I got them through FeatherCraft co. I go through phases doing it one way,and then change for just change sake. I have 17 rods all with reels and lines on them, and set up for different uses so my leaders are tapered near the end somewhat differently on a number of them to accomodate the tippet size, and use for a particular rod, but the "O" ring concept has standardized a few of the rods. I have 4 rods now with the mono furled leader, and tippet ring on them. I am not saying by any means my system is the best, just what I have used over the years. I'm sure you undestand what I mean when I get to .013 and want a 5x tippet. You sure shouldn't tie .013 to .006. I need another piece in there, and use a 10-12" piece of say 2x, or .009. What I want is the last short section to be at least 10" long so when I change a tippet, I don't use up that tapered piece by having it just 8" say. I use a blood knot connection because I can tie it with less than a 1/2" of leader to trim off after tying the knot. If you use the surgeons knot that does connect uneven diameters with a stronger knot you have too much left over to trim off. But the "O" ring solves all of that. If you do buy these "O" rings, I have a tip I an give you.