I don't know this from personal experience because I've never done it, but I've read that with AR15s and .223/5.56 ammo, you can easily load a round that fits into the chamber but OAL is too long for the magazine. IIRC, the person involved was reloading with 77-grain bullets or some such thing. He then seated his bullets to just barely fit the mag.
I'm not a tech expert on these things, but my understanding is that headspace is usually done from the datum line to the shoulder. Based on that, I wouldn't expect bullet seating depth to be a factor in determining headspace.
I also recall that there's a second way of determining headspace, but don't quote me on that.
For me, I got the Hornady headspace gauge kit for my calipers and then gradually adjusted my full-length resizing die from no headspace until I got 0.004" headspace. (Which, when I think about it, doesn't involve the bullet, so that's another reason why I don't think bullet seating depth is involved.)