It depends on what you're making, but pastry flour is usually good for cakes and pastry because it has a lower protein content and so makes for a lighter crumb. If it were me, I'd use pastry flour in a 1:1 substitution for cakes, cookies, and pastry.
All purpose flour has a protein content of around 9-11%, pastry flour is around 8-10%, and bread flour is 12-14%. So if you wanted to make an exact substitution for all purpose flour you could use part pastry flour and part bread flour.
The protein content varies between producers, but if you were to use half pastry flour that had a 9% protein content, and half bread flour that had a 13% protein content, you would end up with a mixed flour with an 11% protein content.
Hope this helps!