In a saucepan, combine sugar, flour, salt and mustard; stir in eggs. Gradually stir in vinegar and water until smooth. Bring to a boil over medium heat, stirring constantly; cook and stir for 2 minutes. Cover and refrigerate. Just before serving, combine desired amount of base with an equal amount of mayonnaise. Serve as a dressing for potato salad, coleslaw or salad greens. Refrigerate leftovers. Yield: 3-3/4 cups base.