Supplementing a 20* sleeping bag

I have 20* sleeping bag and plan to go snow camping. I'm worried that the rating may not be accurate, so I am thinking to make a top quilt out of fleece to supplement the bag.

What do you think... would it help? I plan to use the fleece inside the bag, or would it be better to use it on the outside. I've seen silk liners...would those be better than a fleece?

They claim that a fleece liner bag will give you about 10* of extra warmth inside your bag. Don't know if it's really 10* or not but it does help keep you warmer.