But there are other cases where one is not interested in best performance in terms of turn over. When dry fly fishing, I aim for a leader that's not exactly impossible to turn over, but somewhere in between. I want one that easily lets me present drag free drifts and yet of course, it also needs some sort of performance when it's windy (which it almost always is in Denmark).
A leaders performance is dictated by a number of factors, of course. The caster, the line, the material (nylon or fluorocarbon, I always use nylon) and it's design. When you look at a leader profile, there short story is that the longer the butt section, the better performance in terms of turn over. The short the butt section, there better the leader will be a presenting drag free drifts (basically allowing the leader to collapse to a certain extent).
For my dry fly leaders, I also begin with a pre-tapered leader (to avoid the knots in the thick section). I like long leaders, but "long" has to be seen in the context of the stream you're fishing. Were I to fish are large river in Sweden or Norway, I'd be looking at 15' or longer. In that case, my starting point would be a 9' leader. That gives me a relatively shorter butt section than if I started with a 12' or 15' leader.
For use here in Denmark, where the streams are relatively smaller, my starting point is a 7'6" leader. I usually have to leaders I use for dry fly. One is for May/June, where the large Danicas are hatching. To handle the imitations needed, I never fish thinner than 0,14mm and often 0,16mm. (around 4X, depending on manufacturer). And since out rivers aren't huge I aim for an leader that's around 12-14 feet long. To end there, I choose a 7'6" 0,23mm leader (2X), to which I tie a section of 0,20mm (3X) and a long tippet section of 0,16mm (4X).
For the rest of the season, where smaller stuff hatch, I usually fish 0,12mm or 0,14mm tippets (5-6X), and to make these leaders I use the same 6'6" leader, only a 0,20mm (3X).
You can get really good dry fly leaders in all lengths on the market, and sometimes I do use those, by I like the flexibility I get in using a shorter leader as the base. If I need to shorten a leader up to reach a fish with a roll cast and a big dry, I can do that. And I can prolong it to not spook a fish on a glassy glide, all the while retaining the basic leaders properties. You can't cut 4 feet for a 12' dry fly leader without ruining it's built in properties.
Have a great weekend!