The ratio we used to make pastry flour when I was working with the chefs on Baking with Julia was 3 parts all-purpose flour to 1 part cake flour. Whatever proportions you decide on, just make certain that the cake flour you use is NOT self-rising.

Pastry flour is inbetween cake flour and AP flour in terms of protein content. Cake flour contains b/w 5% to 8%, Pastry b/w 8% to 9%, and AP b/w 9% to 12%. So you can make pastry flour by mixing cake and AP flour in a ratio of 2:1 cake to AP. I think most stores will carry cake flour but if you don't have it you can make it as well from AP flour by taking one cup of AP flour and removing 2 TBS of it and replacing that with 2 TBS of corn starch. Make sure to sift the cornstarch and the AP flour well together.

Here's the substitution offered by JoyofBaking.com, using all-purpose and cake flour, which are both much more widely available than pastry flour. I've used this sub before for pastry and it's worked fine!

2 cups (270 grams) pastry flour = 1 1/3 cup (175 grams) all purpose flour + 2/3 cup (80 grams) cake flour