In a medium saucepan, combine sugar, cornstarch, flour and salt. Gradually stir in water. Cook and stir over medium heat until thickened and bubbly. Reduce heat; cook and stir 2 minutes more. Remove from the heat. Gradually stir 1 cup of hot mixture into egg yolks; return all to saucepan. Bring to boil. Cook and stir for 2 minutes. Remove from the heat. Stir in the butter, lemon juice and zest until smooth. Pour into pastry shell. In a bowl, beat egg whites and salt until stiff but not dry peaks are formed. Gradually beat in sugar until soft peaks form. Spread over pie, sealing edges to pastry. Bake at 350° for 12-15 minutes or until meringue is golden. Cool. Store any leftovers in the refrigerator.