gistfile1.cs

// * Dont need [ImplementPropertyChanged] since it will be detected from the base class
// * Namespaces are noise. dont use them
// * use fielder so you dont need properties https://github.com/Fody/Fielder
// * I prefer property injection. simply less code
// * fields allow me to initialise them outside the constructor

public class FirstViewModel : MvxViewModel
{
  public ICalculationService CalculationService;
  public double Generosity = 20;
  public double SubTotal = 100;

  public double Tip
  {
    get { return CalculationService.Tip(SubTotal, Generosity); }
  }

  public double Total
  {
    get { return SubTotal + Tip; }
  }

}