Once again, I spent too much time writing, and Rami beat me to it by a few minutes. But as long as i've got it, might as well post it... Rami is exactly right, I'm just elaborating. (Personally, I like to hold a string down at the 1st and 24th and measure at the 7th, but there are lots of variations on this - the general idea being to use the string itself as a straightedge.)
oujee,
My first thought was to suggest you go back and do some more reading... but then I realized that maybe nothing posted here explains the basic concept, so I'll give that a try (but once that's clear, then I encourage you to go back and read some more).
First of all, there's no rule that says your neck will be straight when you loosen the nuts to just finger tight. *If* you took the strings off, it might be close, but just forget about that for the moment.
Think of it this way. Suppose you have a long piece of slightly flexible wood, and on one side it has a set of strings, and on the other side it has a truss rod or two. If you tighten the strings, the wood will bend towards the strings; if you tighten the truss rods, the wood (neck) will bend back towards the rods.
If the wood started out straight, and you applied exactly the same amount of tension to both the strings and the rods, then the wood would remain straight.
If you switched to strings with higher tension, then they would try to bend the neck more, and you would want to tighten the truss rod to compensate; with lower tension strings, that were exerting less forward bending pressure on the neck, you would need to loosen the truss rods so that you didn't get a reverse bow. And even without changing strings, sometimes the neck itself will shift as a result of humidity changes and so forth.
In other words, whenever you tighten the truss rods, the neck will first get straighter (assuming it was already bowed forward), and then eventually start bending backwards - but hopefully you won't go that far.
You have to check the relief with the strings tuned to pitch, because they make the neck bend forward, and this is the force you want to balance with the truss rods.
As a general rule, it is not a great idea to loosen the nuts to finger tightness and then work from there. For one thing, there's no guarrantee that both rods will require exactly the same number of turns. Or to put it another way, if both nuts were just finger tight, you might have more relief on one side of the neck than the other (though usually it would be close).
Secondly, you don't really want to make major changes to the truss rod tension all at once. It takes a while for the wood to adjust, so some people suggest you don't go more than about a quarter turn in one day, or at least let it sit for a few hours before deciding whether you really need to go further.
But if you've already loosened them, don't panic. Alembic seems to deliver their instruments so that they require very little truss rod tension, and unless the wood has shifted over time I suspect it would be rare that you'd have to go much more than a half turn for them to become loose (often less).
Start by comparing the relief of the highest and lowest strings. If they're not close to the same, then adjust one of the rods to make them so. Once they're close, then you would generally adjust both rods evenly, in the same direction. If you have too much relief (bow), then tighten the rods. If the neck is too straight, loosen the rods.
Just think about it a little, and it should be pretty clear. Good luck.
-Bob
