You’ll get a couple of degrees with a fleece or wool. Not much, and I’m not aware of a chart. There is a chart through Enlightened Equipment that shows what stacks to what when quilts/bags are stacked.
Boil a Nalgene-full amount of water right before bed and slip the Nalgene into an insulated bottle sleeve. When you wake up cold in the middle of the night, you can unzip the bottle warmer and the bag will get nice and toasty until dawn.
@Storm
I have found that slipping the bottle into a woolen sock is perfect. It will insulate it enough to radiate just the right amount of heat the whole night but won’t be too hot to touch right away when the boiling water is poured in.
Insulation is cumulative, and the more air you trap, the more insulation you get.
So the thickness of a fleece liner adds as much air and insulation as that fleece can hold. If it’s 2mm thick, it’ll be about as warm as wearing a thick fleece top and bottoms.
Adding a quilt over your bag will yield better results because it can be thicker and hold more air than a liner.
Down is the lightest for its warmth (assuming the cover is lightweight, low denier polyester or nylon).
For an extra inch of down on top (700 or higher FP), you can expect to get an extra 10°F worth of insulation.
Pair that with extra insulation under you by adding a 7mm or thicker closed-cell foam mat (on top of an inflatable pad if that’s what you use).
I wouldn’t trust any chart as a definite answer and even less claims made by a manufacturer. For example, I have a Sea to Summit Reactor Extreme, which they advertise increasing warmth by ‘up to 15°C,’ and I can safely say that is complete bullshit unless your sleeping bag has a comfort rating of +30°C. I slept with my +10°C comfort sleeping bag in +5°C and woke up feeling cold a couple of times during the night.
So my tip is to test your setup before you go at your home yard/balcony/nearby forest to determine if it will work for you. Also, the Nalgene hot bottle trick is a very nice backup in case you don’t have the possibility to test your setup beforehand in similar temperatures. That way, if you wake up cold, you can easily add extra warmth and not sleep miserably during your trip.
Adding more insulation underneath you is the best way to add comfort. Next, be sure to change clothes with a fresh base layer. The issue with adding a layer inside the bag is it may compress the bag’s insulation. When you lose loft, you lose warmth.