While putting together some NuGets for some portable class libraries I found my self a bit lost of what target string to use. One way to find some guidance is the use the NuGet Package explorer to build up the NuSpec, but I found another source, which felt like a real life saver. Have a look your self and note the NuGet target column in the table.
Saved my day, hope it helps you as well,
Cheers,
//Daniel