I hope this isn't too long but you asked a lot of questions. I also note that others have chimed in while I was typing this.
The headspace of a case means the distance from the head (bottom) of the case to a 'datum point' on the shoulder. This datum point is the point where the diameter of the shoulder is exactly 0.40" for a .308 case. The headspace of your chamber is the distance from the bolt face (w/ the bolt closed and locked) to the same point on the chamber's shoulder. Obviously, the case headspace must be shorter than the chamber headspace or the round won't chamber and the bolt won't close.
If you run your brass through a standard full-length die adjusted like the instructions tell you (screwed down into the press until the bottom of the die hits the shell-holder on the ram), it will usually set your case headspace to the SAAMI minimum, usually much shorter than a typical chamber's headspace. When the round is fired, the brass will stretch in length until the case shoulder hits the chamber's shoulder. Then when you resize the case, the shoulder gets shoved back to minimum again. This stretching and shoving will cause your brass to wear out sooner than need be, and this is what people are talking about when they talk about adjusting your case headspace to fit the gun. If you back your sizing die out a bit, you can lengthen the case headspace and thus reduce the distance the case shoulder will stretch before hitting the chamber's shoulder. This will make your brass last much longer and greatly reduces the chance of a case separation. It also helps with accuracy.
EDIT to above: Actually, what happens when you fire the gun is this: As the firing pin is striking and igniting the primer, it also drives the round forward until the case shoulder hits the chamber shoulder. Most of the stretching takes places back toward the rear of the case as the pressure drives the head back until it hits the bolt face. That's why case separations usually take place back by the head and not up by the shoulder.
In order to determine how far to back out your sizing die, you need to know what your chamber headspace is. And you need a measuring tool or two to help you gauge the headspace of your case. I'm familiar with the RBCS tool you mentioned but have never used one. I use a
Hornady Lock-N-Load HeadspaceTool Set w/Bushings and the Redding Instant Indicator.
Finding the chamber headspace can be a problem because there's not a tool I'm aware of that you can stick in your gun to give you that reading. With some custom-fitted guns the maker can give you the headspace, but usually not. (Even if they give you a headspace measurement, I like to find my own. Sometimes different tools and methods yield different measurements.) What we do instead is measure the headspace of cases that have been fired in that gun. With the M14, you have to turn off your gas valve when doing this because the case can get stretched during cycling.
If I fire 10 new cases in a given gun and then measure the headspace of the brass when it comes out, I will typically get several different readings all within a small range. Too many factors to go into to explain this, but the longest case is the closest to the chamber headspace. It usually takes several firings (with neck sizing only in between) for a case to fully conform to the chamber size, and this is the best way to get an accurate reading. Taking a measurement after one firing is better than nothing.
The goal now is to set your sizing die so that it sets the shoulder on cases just under the size of the chamber headspace. Depending on whether you have a bolt gun or a semi-auto, the target length is in the neighborhood of .001 to .004 shorter than the chamber headspace (more for a semi in order to insure reliable cycling, less for a bolt gun).
The Hornady and RBCS tools will give you a numerical value for the measured headspace. The Redding tool gives a comparison but not a hard number. The Redding is much faster to use than the other two. You could get by with just one tool here.
I have five .308 guns and each one has a different headspace. The longest are two M1As and the shortest is a tight-chambered GAP custom bolt gun. Once the initial measuring is done, all I have to do now is set my sizing die differently for each gun. I know what my Redding Indicator reads for each gun so I just use that to compare cases coming out of the sizing die to make sure they have the target headspace for each. The ammo for the M1A won't even chamber in the GAP, so I have to keep my loaded ammo separate and well-marked. Using a different brand of brass in each gun makes it easier to sort through fired brass. I use Lapua, Win, IMI, LC, and Black Hills, with some Fed thrown in here and there. If I lose track of which brass is which, a quick run through the indicator will tell me. It's generally bad mojo in terms of accuracy and case life to swap brass from gun to gun, but I do have a stash of SHTF ammo that will work in any gun.
The Redding tool comes with a 'standard' case that lets you set the dial indicator to zero. I keep notes on how far away from zero each gun's brass should be. This way, if I use the tool for one of its other purposes, I know I can go right back to the same zero when I need to compare headspaces.
As far as OAL and seating depth goes, you are usually limited by the length that will fit into your magazine. Sometimes you can stretch your bullets out till they touch the lands for better accuracy, but that turns any gun into a single shot. I have a Hornady tool for finding what length of any given bullet will touch the lands, but I usually just use a case that has a slot cut in the neck with a Dremel tool. It will hold the bullet but allows it to slide back into this cut neck when I chamber it in a gun.
I learned my basic loading (pre-internet) with a Lyman reloading manual. The best book I've read is Handloading for Competition by Glen Zediker, but it is very difficult to read - partly because of the technical info and partly because of his rambling writing style. Another source is to go to the reloading section of the forum at Sniper's Hide, then look for the series of stickys there. But beware that these stickys are aimed at the super-anal bolt gun shooter looking to eliminate any source of inconsistency for long-range shooting.
