I completely agee with TinaLouise.
Test - Test - Test
About once a month [which is as often as my wife will let me], I will sleep in my backyard to test different sleep systems.
I had pretty much figured out what what combinations of tent / bag / pad / clothes worked for me down to 0 degrees.
Now I am doing it all over again with a hammock .
A couple of things:
you are usually dry, well rested, and well fed when backyard camping which might not be the case when in the woods - that makes a huge difference, so be conservative with your sleep system when you go out for real
one of the biggest surprises for me was that "cold is not linear" - i.e. the difference between 10 deg F and 20 deg F is greater than the difference between 20 deg F and 30 deg F AND the difference between 0 deg F and 10 deg F is greater than the difference between 10 deg F and 20 deg F - what I mean is that something that extends your system at 10 deg F when sleeping at 30 deg F might have a little effect when sleeping at 0 deg F