Qualified Education Expenses under 529, dorm versus apartment

Figuring out the qualified 529 expenses for room and board was easy when living in the dorms and just having a meal plan. But now that the student is in an apartment I believe we are limited by the room and board numbers under Estimated Cost of Attendance. But the school comes up with its number based on the school year, and my taxes are done on a calendar year. So each year, how do I figure out what the maximum is for each calendar year? Obviously if the actual expenses are lower then we would use the actual expenses.

You’d take the expenses for Sept-Dec in this tax year, the rest in 2017.

I’m clear that I take the expenses in the calendar year in which they are incurred. But my question is how I calculate what the maximum amount of allowable expenses (if they exceed the Estimated Cost of Attendance).

I use the 2015-2016 winter semester and 2016-2017 fall semester cost. And I withdraw money per semester.