gistfile1.cs

public class Formula<T> : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged = delegate { };

    private readonly Func<T> formula;
    private readonly INotifyPropertyChanged source;

    public Formula(INotifyPropertyChanged source, Func<T> formula, params string[] dependencyProperties)
    {
       this.source = source;
       this.formula = formula;
       this.source.PropertyChanged += (sender, arg) =>
       {
          if (dependencyProperties.Any(p => p == arg.PropertyName))
          {
              PropertyChanged(this, new PropertyChangedEventArgs("Value"));
          }
       };
    }

    public T Value
    {
        get
        {
           return formula();
        }
    }
}