Use the processor to cut in the shortening, then finish the job by hand to avoid adding too much water. This type of pastry is generally appropriate for most pies.
Put the flour, chilled butter, and shortening into the processor bowl. Pulse to cut in fats to the size of split peas. Add lemon juice or vinegar; pulse a couple of times, add the ice water in dribbles, pulsing the machine just until the dough is a shaggy mass. Turn this out onto a floured surface and push it away from you with the heel of your hand to blend lightly. Add more ice water if necessary to make the ingredients hold together. Shape into a ball quickly and flatten the mass. Wrap in plastic and refrigerate an hour or overnight.
Let the pastry warm up slightly so that it can be worked. Place it on a lightly floured board or cloth and roll out, turning the circle after each two strokes until it is two inches larger than the circumference of the pie pan. Gently lift it into the pan and press it lightly into place. Trim the edges, leaving a half-inch overhang. Fold this under a form a rim and crimp the edges. Prick the pastry all over.
Preheat oven to 375°F (190°C). Put the shell in the freezer for 10 minutes. Remove the pastry shell from the freezer and line it with foil and some dried beans or rice. Bake on the lower oven rack for 15 minutes. Reduce heat to 350°F (175°C). and remove the weights and lining. Continue baking for another 10 minutes or until dry and firm. Cool thoroughly before filling.
I prefer all short pastries to be twice-baked. Though there are many traditional recipes that begin with raw pastry, I find that all custard pies have soggy bottoms if the shell isn't baked, once "blind" (empty) before filling and then the final baking.
Tip: Make extra pie crusts to freeze in foil pans so they needn't be defrosted before baking.