To answer the questions that you ask: a usual recommendation to highpower target shooters who want to turn necks to fit their chambers, is to allow .001" clearance on each side - so if your fired brass is .345", you would arrange for the loaded round to be .343". Note that you should measure the diameter at different locations on the neck, often there is .001" or .002" taper along the neck. Competition benchrest shooters will sometimes set their brass up to be an even closer fit than that mentioned above. This tends to be a lot of work, perhaps an hour or more per case by the time the dust settles; then again, since a BR shooter only needs twelve or fifteen pieces of brass for his rifle, if this gives even a slight advantage it can be worthwhile.
It does sound like a neat, fun project. Go ahead and do it, if it's something you'll enjoy doing. I have run .30-06 cases into a .308 die, I never have finished the job by trimming it to length and turning the neck diameter.
But to answer perhaps what you're actually interested in, it's most unlikely to be of any help whatsoever for improving the accuracy of your .308. I'm not aware of any instance (factory chambers or custom chambers) in which having reduced clearance between the neck and the chamber will help accuracy. Fitted necks are done in the Benchrest game, but for other reasons.
from what i saw it allowed the user to get a case with a thicker neck that could them be turned to precisely match the chamber neck for better concentricity when chambered
That's why people do it, and that's what they think is happening. But that isn't what happens. A cartridge case is a series of tapered surfaces, and it fits into a chamber which is also a series of tapers. The case gets pushed forward until the (tapered) shoulder of the case bears against the (tapered) shoulder of the chamber. If the case is held firmly forward (and in any rifle which has a spring-loaded plunger ejector, it is), then the font part of the cartridge case is held centered in the chamber. The neck walls will not be touching the chamber neck area, but the bullet will be held in the middle of the chamber, pointing down the middle of the bore (which is what you want).
(This assumes that the ammunition is made properly - the case neck has to be concentric with the rest of the cartridge body, the neck has to be straight, and the bullet has to be seated straight in the neck. If all of this is done, then when you measure the "runout" of the loaded round (roll the round and measure how much the bullet "wobbles") will be small, i.e. within a few thou of perfectly straight. If your ammo has too much runout, say .006" or .010", you can start looking to see what the cause is. Usually it is caused by the sizing die making the neck crooked, or by the seating die putting the bullet in crooked).
So if your ammo isn't shooting as accurately as you like and as accurately as you think it ought to (for the quality of the bullet used), perhaps one of the first things to check is whether or not it is straight (borrow a gauge, and measure the runout of several of your rounds).
