Two things I'd suggest. If so, then perhaps you do need to lower that rating a bit further with the addition of a vapor-barrier liner. By reducing the cooling effects of evaporation from your skin, they make you warmer. As with all things involving comfort and temperature, it will vary. But easily five degrees Celsius, I should think.
Usage is simple.
Just put the thing in the bag and crawl inside. No need to change what you wear, or wear less or more. The only concession you may have to make is to keep a dry pair of long underwear handy. A VBL traps moisture in the bag, so you may wake up with damp underwear. It will dry quickly on your body, but will still feel a bit clammy first thing.