Whats the chance my cockatiels eggs will be fertile?

I have a pair of cockatiels who have been dna sexed. They are 2 years old. They have been mating for at least a week but everytime I see them mate the male has his tail by her wing and sometimes head, but their in their own room so they have their privacy. The female stands on a purch attached to the nest box so its hard for the male to position his self. The female has been staying in the nest box more lately and her droppings are huge beyond belief! And she has almost finished a big cuttle bone. She gets a variety of foods. What's the chance of the eggs being fertile if she lays eggs?