With most inexpensive sleeping bags, the ratings are pretty fictional--best to assume the real rating is 5-10*F above the advertised rating.

There are standards for sleeping bags sold in Europe, but even those are a bit iffy. Info on EN13537 sleeping bag standards. Note that there are several different rating standards. All the ratings assume that (1) you are "standard," whatever that is; (2) you are wearing a base layer top and bottom and a warm cap; (3) your sleeping pad has an R value of about 4. A few US companies have adopted the standard. A number of US companies don't use these ratings--testing does cost money and the ratings themselves have been questioned (supposedly, testing the same bag in different labs produced highly variable results).

Most of the expensive high quality down bags are pretty accurately rated: Western Mountaineering, Feathered Friends, Nunatak, Montbell. These companies all have a reputation to uphold.

Putting tarps around your tent will just result in a lot of condensation inside. A wet sleeping bag is a COLD sleeping bag!