PREHEAT OVEN TO 200ºC . Make the crust, line a pan and roll out the 2nd crust. Refrigerate both while you make the filling. If you have the larger amount of grapes, use a deeper pie tin. Slip the grapes from their skins, working directly into a sauce pan. Put the skins into a bowl. Cook the grapes until they turn whitish and the seeds are loosened, about 3 minutes. Pass them through a food mill to get rid of the seeds, working directly into the bowl with the skins. Stir in the lower amount of sugar, the lemon zest and the flour. Taste and if more sugar is needed, add the rest. Add the lemon juice, to taste. Let cook for 15 minutes. Pour the filling into the shell and cover with the 2nd crust. Crimp the edges together well so that the juice doesn't leak out. For a more golden crust, brush the top with a little beaten egg yolk. With the tip of a knife, etch a decorative design over the top. Or cut some grape leaves out of the scraps of dough and fasten them to the crust with a little egg yolk or water. Bake for 10 minutes in the lower 3rd of the oven then reduce heat to 180ºC . Bake another 35 minutes or until the crust is nicely browned. Cool before serving.