There are a couple unaddressed variables that make a huge difference.

The first is what rating that is. If it is just the vague general rating that many manufacturers sometimes claim it may mean next to nothing. If it is and EN rating that is more useful, but even then there are different numbers in the EN rating system like "EN Rating: T-LIMIT" or "EN Rating: T-COMFORT".

My favorite bag has a nominal 45F rating and if far warmer than a few 32F bags I have tried. I find it warmer than some 20F bags I have tried.

Second I find that individuals tend to sleep warmer or colder, so even with the EN rating you need to consider the individual. The difference between individuals can be pretty extreme. You can expect bags with similar EN ratings to be similar, but it is hard to predict what rating an individual needs for a given set of conditions.

I apparently must put off heat like a furnace because I have used and been happy with my Mountain Hardwear Phantom 45 in temperatures down into the teens F and often get up comfy and warm after having never need to close up the bag tight, only to find everyone in camp complaining how they froze in their colder rated bags.

As far as things to extend the range... Wearing a layer or layers of clothing works well if it doesn't cause the bag's insulation to compress due to a tighter fit. Just piling clothes on top works well too. Keeping the wind off with a bivy or tent makes a big difference.

One caution... Being very warm early in the night may be counter productive especially if you get damp with sweat. I find it better to adjust as the temperature drops. Start out with the bag just loosely draped over you if it isn't yet cold. Progress to getting in the bag, then zipping the bag partway, then att the way, and finally pull the hood drawstring tight (but only breathe through the small opening, not into the bag). I find that taking in hot water bottles is counterproductive because it creates heat early before it really gets cold and cools off when you need the heat.

The very worst approach is to get all toasty warm early on and then get damp with sweat. If I do that I find that I get cold later. I find it better to sleep slightly cool at first and add warmth as the temperature drops, but never to the point of getting even slightly damp.

One other thing... If I need to urinate I feel much colder. The tendency it to avoid getting out of the bag and into the cold, but I find that I actually feel warmer after getting up to pee. Some folks take a bottle to bed to avoid the need to get up during the night. That has worked well when I have done it, but I have not made a regular habit of it.

The thing is that you need to figure out what works for you. I know that most of my friends say that what I do would not work at all for them. Until you figure that out, remember that you can wear all of your clothing in a pinch.

Oh and one other thing... I find that a pair of warm and DRY socks is a huge help when it is cold, at least for me. Don't use the damp ones you had on all day.