Why do we use predicted overhead costs / direct material costs?

To determined the overhead rate, don't I have to use either direct labor hour or machinery hour?

In December 2016, Custom Mfg. established its predetermined overhead rate for jobs produced during 2017 by using the following cost predictions: overhead costs, $750,000, and direct materials costs, $625,000. At year-end 2017, the company's records show that actual overhead costs for the year are $830,000. Actual direct materials cost had been assigned to jobs as follows.

Jobs completed and sold

$513,750

Jobs in finished goods inventory

$102,750

Jobs in work in process inventory

$68,500

Total actual direct materials cost

$685,000

Question: Determine the predetermined overhead rate, using predicted direct materials costs, for 2017.

Solution: predetermined overhead rate = overhead costs / direct material cost = 750,000/625,000=1.2 A. 120%