well, headspace is somewhat tricky but very important issue. Here's my take on it, i'm no expert so please someone else chime in.
you say you're reloading, are you full length resizing or neck sizing? Anyways, back to headspace. headspace on a cartridge is taken off a reference line, or datum line. This "line" falls somewhere on the case's shoulder. The datum line on a 308 family case is .400". So where ever the shoulder of the case opens to .400", that is the headspace, as measured from the case head to that point on the shoulder. In the chamber of your rifle, you would measure from the bolt face to the spot which the cartridges' shoulder would hit, at .4". There are 2 measurements of headspace, one taken off the cartridge, and one a gunsmith would take, inside your chamber. When a gunsmith chambers a rifle, he has to make sure he leaves enough room for a round to be chambered in the gun.
How to measure headspace: I use a stone point headspace comparartor. It attaches to there bullet comperator, and is used the same way. Once you put the right bushing in the bullet compararator, you can then measure your cartridges headspace. You can find them at midway, sinclair, etc. fairly cheap tool, easy to use. I asked if you neck size or f/l resize. If you neck size w/out "bumping" the shoulders, pretty soon one day your going to try to chamber a loaded round and the bolt won't close. Or maybe you'll find that bolt is getting harder and harder to close. basically what's happening is after every time you fire that case, the headspace is getting a tad longer. Take some measurements on new brass, and then fire it & reload a couple times. you'll see that cartridge is getting "longer", but not oal, just from headspace. another words trimming wouldn't solve the problem, because trimming does not set the shoulder back. Basically, you don't want to let your headspace get so long that it interferes or causes problem with you bolt & chamber dimensions. If your full length resizing, normally not so much an issues. If your neck sizing only, than itcan become a problem.
Sorry, didn't mean for this post to get so long. There's alot more to it than what i posted, I hope it helps. take the time to learn about it. If not checked, excessive headspace can lead to excessive pressures and stuck bolts.
just my .02