Appium automation query on selecting book from the list

Hi All,
Have a small query on book list and select the book from the list .
Scenario is
1.There is list of different books in tile view.Navigate to this page
2.Scroll to the bottom of the page(the list of books has 20 book list) last third book from the list from bottom.

I am taking total count from the list and tryimg to click on last third element from bottom but its not working.

Please help .

share your code maybe we can help.
You should get the list size and then calculate what index the last third element has and click on it