If you aren't already aware of it, most sleeping bag ratings are overly optimistic. (Except for military equipment.) My 45F bag is useless below 60. Had to stop at a Wallymart and buy a fleece bag to put inside of it. It's a good thing I did that, we had frost on the bags the next morning.
That is widely variable buy individual person, by how optimistic the rating is, and by how much it is supplemented by being in tent or bivy and how much clothing you wear or pile on top of yourself.
If you go by the EN ratings they should at least be consistent. FWIW, I find my Mountain Hardware Phantom 45 to work out fine for anything but real winter camping. I sleep on top of it if it is hot and add more clothing as it gets colder. It has worked out well for me from hot weather down to the teens F. A cap and a really warm loose fitting pair of socks helps immensely. So on top of a pass with my thermometer reading 18F I was still comfy enough. The locals claimed it was actually colder. I apparently put out heat like a furnace though so YMMV.
It helps to not bundle up too heavily too early and get sweaty, better to adjust to the temperatures as they drop during the night. Adjustments can be made by whether you drape the bag over yourself, zip it up, put the hood on, and pull the hood drawstring tight leaving only a small breathing opening. Don't breathe into the bag or you will get it all damp and be colder.
You really need to find what works for you. Advice from me, or anyone else may be spot on for them and way off for you.
Oh, and let me say again that ratings other than EN ratings are not to be trusted or even given much consideration at all. I have used nominal 32F and even 20F bags that were not as warm as my Phantom 45.