What you are asking for is simply impossible.

If you want to make nettle juice for feeding, all you need to do is make a tea as in "tisane" - just leave the cut nettles in boiling dechlorinated water 1-2 minutes. If you pick them from your garden, maybe you don't need the NPK so badly because they are feeding on the same soil you are growing plants on but this is again uncertain.

Nettles are reputed also for taking polutants from the soil (so don't pick your nettles near the road).

If you forget the thing fermenting for more than it was said (a few weeks or a month more) then water evaporates a lot, the composition changes and you would need to dilute it a lot more.
The link I am posting below gives many other recipes for plant. The nettle recipe is translated from there.

Nettle tea recipe:
Cut roughly 1 kg of nettles for 10 litres of water. Let them soak 12 to 24 hours (at 18°C) for an insecticidal and fungicidal effect

leave 10 - 15 days at 18°C (when mixture does not produce bubbles anymore when gently mixted) to serve as fertiliser and growth stimulator:
Dilution
5 %: for leaf sprays
10-20 %: sprinkling at the foot of plants
Usage: Rich in nitrogen, organic and mineral elements and in trace elements, constitutes an efficient fertiliser
Precautionary treatment against mildew, rust, oïdium
Repellent of dust mites, aphids

COMFREY
It is the most precious green manure in the kitchen garden. Rich in nutritious elements (particularly nitrogen and potash), it is used to form a tea as powerfull as the nitrate of potash.
about 1 kg of leaves is left to macerate for one week in 10 litres of water. It is filtered and then sprayed on the foliage (diluted in 1/20) or used to water the plants (diluted in 1/4).

references:
(http://groups.google.fr/group/fr.rec.jardinage/browse_frm/thread/b83a51a0e61674f/ba390ed6e1d9ca96?lnk=gst&q=purin+orties&rnum=1&hl=fr#ba390ed6e1d9ca96)

Apparently, everyone on fr.rec.jardinage agrees that nettles are high on N so too much of it could favor disease.

I found a scientific reference for the effect on nettle juice as immunity booster but it is meant for human usage (research done on mice to check for possible use on humans) not on plants. For plants, i couldn't find much.

http://cat.inist.fr/?aModele=afficheN&cpsidt=200816

The article is in French but you can use an internet translator to have an idea of what it says.

Thanks for the feedback, some useful, some not. The comfrey liquid produced from putting the leaves in a plstic barrel and draining off the liquid, has a N.P.K. of 8 - 2.6 - 20.5
I was wanting the N.P.K. for the same liquid made with nettles, assuming that the nitrogen reading will be higher, I can the feed my veg. organically, using each liquid at different rates accordingly.

Hi, I am involved in a small company that manufactures highly concentrated non-smelly comfrey and nettle liquid extracts for gardeners and growers. For both these plants our extracts come out at an NPK ratio of about 3.