Place oven rack in middle position with a second rack below. Heat oven to 425° F. Prepare pastry dough. Roll out bottom crust and line 9-inch pie plate.
Mix sugar, flour and grated orange peel. Turn half of rhubarb into pastry-lined pie plate; sprinkle with half of sugar mixture. Repeat with remaining fruit and sugar mixture; dot with margarine. Cover with top crust that has slits cut in it; seal and flute edges. Brush top of pastry dough lightly with milk and sprinkle with sugar, if desired. Cover edge with 2 to 3 inch strip of aluminum foil to prevent excessive browning; remove foil during last 15 minutes of baking.
Line a baking sheet that is wider than pie plate with foil. Place on rack below one for pie to protect oven from overflowing pie juices. Place pie on rack in center of oven. Bake 30 minutes then remove foil. Continue baking an additional 10 - 20 minutes, until crust is brown and juice begins to bubble through slits in crust.