I rent rooms and a car garage same property but garage rental not in connection with room renters. How do I prorate the garage if using number of rooms instead of S/F?

It is a detached 4 car Garage + part of the driveway, utility in the garage is not separate from the house, 100% rented no personal use. 

How driveway kicks in? It is rented separately as well not with the garage. 

it is a big  garage yet prorating square footage tax wise would not be to my advantage x rooms.

Thank you