This is what worked best for me:
string parentOfStartupPath = Path.GetFullPath(Path.Combine(Application.StartupPath, @"../"));
Getting the 'right' path wasn't the problem, adding '../' obviously does that, but after that, the given string isn't usable, because it will just add the '../' at the end.
Surrounding it with Path.GetFullPath()
will give you the absolute path, making it usable.