What is the proportion of fat to flour in flaky pastry?

The proportion of fat to flour depends largely on the type of pastry dough you are talking about, and what your fat source is. For pie crust dough, I've seen the ideal ratio described as 1 part fat to 2 parts flour. However, that ratio applies just to the ratio of one ingredient to another, not to the ultimate percentage of fat involved.

Butter and shortening, for example, are not equivalent, and don't have the same fat content: shortening is 100% fat, whereas butter is around 80% fat (and the fat content can vary by brand).

If you were referring to actual pastry dough, the percentage of fat to flour is going to differ more greatly. A popover dough for example, is going to contain a lot less butter than a pastry based on a puff pastry dough, Croissants, for example.