Below, is my formula for a long leader that works.

It is designed for the following application:-

Wild river for very spooky fish, who will take fright at the first hint of a flyline splashing on the water, and for the same fish, which won't tolerate even the stealthiest of wading.
It is designed to be fished primarily accross the current, to an undercut bank or structure that hold fish, over flowing riffle water or currents that would cause unnatural drag on a line that lies on the water. It is designed to do away with line mending and any other technique that would involve any line touching the water.
It is best fished with a long rod, minimum of 10 foot, with a soft middle to tip action, rated for a line weight of 3 or 4.

The method it is fished, is as follows:-

cast the line out towards the target area, with a flick of the rod tip, using as little false casting as possible, preferably none, with a maximum of a meter of fly line out of the rod tip. The softer the rod the better, as you will be unlikely to be able to feel the rod load at first, until you have a bit of experience with the leader.
The flies would typically be two nymphs of light to medium weight. As soon as the flies land, the rod should be lifted so as to allow as little as possible of the leader to touch the water. The flies should land on the water before anything else, and the rod lifted immidately to prevent any line touching the water. The leader will stay suspended in the air, between the flies and the rod tip, due to the lightweight nature of the leader, where a suspended flyline would fall to the water surface due to it's weight. Allow the flies to follow a controlled drift through the zone, and once they have drifted out of the zone, the rod is gently lifted to allow the water to cause a minor loading of the rod. the rod is flicked back to aerialise the leader, and flicked forward, again to the target area, and the process is repeated.
Note that at no stage does the line run through the guides of the rod. The fully extended line is cast at the fishing distance. in other words, if the fish are 30 feet away, and that is the distance of the cast, the full 30 feet are managed with one back and forward flick cast. (It's a completely different way compared to aerialising fly line)

The last 1 meter of terminal tippet, is de greased, to allow for as quick as possible sink.

Formula as follows:-

0.60 ---- 4m
0.55 ---- 2m
0.50 ---- 1m
0.42 ---- 50cm
0.37 ---- 50cm
0.35 ---- 50cm
0.30 ---- 40cm
0.25 ---- 30cm
0.22 ---- 30cm
tippet--- 1m (6 or 7X)

This leader is roughly 30 feet, which would be the ideal practical length for this type of fishing, however, if it is needed to be longer, a legth can be added to the butt section, in 4 meter section, of 0.65 diameter , and even a further 4m section of 0.70. It is rare that it would be required to extent the length, but in some cases, it can be required. The leader will still turn over if extended properly.
The beauty of this leader, is that it will turn over at shorter use as well, so if you cast to a fish that is closer than the 30 feet, you simply have to wind the reqiuired length onto the reel, and cast it with a shorter section out of the rod tip.
The leader will also be straight between rod tip and flies, and will give you good feel when a fish takes it even with the softest of touches.

Method of construction:-
join the section together, the thicker sections by means of a blood knot, and the thinner, by means of a three turn waterman knot. Do not trim the tags.
Boil the entire leader for a few minutes, and remove from the boiling water, and pull the knots tight while the leader is still warm, and trim the tags.
An reasonable quality supple mono will work, I prefer Maxima.

This method could perhaps be likened to a type of Tenkara style, but with conventional fly tackle. and at a further distance.
This method described above is absolutely deadly for extremely spooky wild fish, holding in shallow water.

Hope it is clear enough, and I will answer any questions that might come up.

Very interesting Andreb, I would have thought that casting 30ft of leader must be nigh on impossible! I presume that the fly is reasonably heavy to aid the casting? Out of interest why do you prefer to construct your own leader rather than use the specially made tapered leader such as the one's made by Hends?

__________________
Fundamentally fishing is a philosophy. A philosophy of earth, and growth, and quiet places. In it there is a rule of life, a recognition of permanences. It makes you notice the little things of nature, wherever you may be. ~Bernard Venables

Any problems with the knots of the thicker end sticking to the guides when landing a fish?

Yes, It can take some practice to bring the knots through gently, but a little bit of Loons, UV knot sense, makes a huge difference. I have seen no ill effects on the rod gudes. Good knots can be surprisingly unobstructive.

Casting the leader is surprisingly easy, as long as you don't try casting it with too much aerialised fly line. It is the leader that is designed to give the distance, and not the fly line. It works best with no fly line out the tip. The leader works best with one back and one forward flick. We test these leaders with nothing attached to the end, so , no, there is no heavy fly to carry it out.
To answer your question about why I don't use pre made leaders like Hends, well, I don't know. I have not tried the 9m Hends, but I hear that it is good, and used to great effect on the European mainland. this formula comes from a combination of the French and Spanish techniques, that they use successfully on shallow clear rivers, where the fish are ultra spooky, and they fish at long distances without any line lying on the water. I have always constructed my own leaders, and to me, it's as much a science as tying flies.
This formula, I must say, is not my own, but comes from the World Championship team from one of the Central Easten European countries.

with so many knots it should be ok to be cast with nothing, with slightly heavier flies I would like to have less knots on the leader

I often use 15 foot pre made tapered leaders and add tippet to them. Thy cast easily with smaller flies its heavier flies that don't seem to work so well. Or maybe that just my bad casting

Very interesting. Are you casting downstream with a helping wind? I had go trying to cast a Hends tapered leader upstream when there was a slight downstream wind and it was useless! The videos I have seen of people casting/fishing with this technique it seems like the cast is more of a lob (hence the question about whether you use heavy flies). I think I need more practise to get used to the long leader.

__________________
Fundamentally fishing is a philosophy. A philosophy of earth, and growth, and quiet places. In it there is a rule of life, a recognition of permanences. It makes you notice the little things of nature, wherever you may be. ~Bernard Venables

As a newbe to fly fishing I have read about and tried most of the methods-as you do!
I’ve tried this very long leader “lark” and it does work-meaning- I can cast it-but the conditions need to be perfect. Example= If I am standing in the water looking upstream, the wind is upstream ,I want to cast upstreamish, and I am able to let the whole leader drift downstream from me to give a bit of loading on the rod, I can cast the whole 25ft+ of leader no problem.
If the conditions are not absolutely perfect to what you want to achieve---- leave the whole set-up in your bag and use another method.
Only been fly fishing for “5minutes” and here’s me giving advice.

I bought some 9 metre fluo tapered leaders in the CZ just before Christmas that I haven't used yet. Shop owner (a pal) said they were easy to cast with without any fly line out