I assume the brass you have was fired from that actual chamber?

I guess it depends on a few things. For instance, when I built my Creed, I purposefully set it tight on a go-guage as I said, to minimize case growth with factory loads. I set it so tight that it will barely chamber a factory bullet. I didn't realize that at the time, and now I don't shoot anything but handloads anyway, and I'm not going to sell the gun to anyone, so it's a moot point. I have the tools to measure case headspace (Hornady), so I just set my full length die to bump the shoulder .001-.002" and I'm good. If you're in that same boat, don't plan to sell it, and plan to handload for it, I'd say you'd be fine setting the headspace with a fired case from that chamber. I'd set it fairly snug on that case, then you'll just set your dies up to bump the shoulder .001-.002" and roll with it.

Now if you could sell it one day, or there's a chance someone else might try to shoot anything factory, or otherwise, through it other than the custom handloads you would be working up for that particular rifle, it may not be the best idea, at least not in my mind. Again, all depends on your particular circumstances.

I'm sure I rambled here, but hopefully it was understandable, and hopefully it helped.