The furnace and water heater are often installed in the middle of an unfinished basement, with access to the entire volume of air in the basement for combustion. Once a basement is finished, however, the furnace and water heater are usually isolated in a small mechanical or utility room, which limits the available combustion air and requires a different strategy.
When the mechanical room is adjacent to an outside wall, openings for combustion air can be direct to the outside.
Remember that air just needs to be introduced to the basement or mechanical room, not brought directly to the equipment.
In colder regions, air brought directly to a natural-draft appliance can have an adverse effect on the flue, and the cold air can freeze nearby water pipes.
Calculating the weighted values for the two methods and summing them to comply with code can be tricky. I suggest buying the Combustion Air Calculator, a $0.99 app from codecalculators.com, which makes easy work of the calculations for all the methods, including sizing air openings and grille reductions for free area.
Appliance location also makes a difference when you're planning for combustion air. If the air volume provided for combustion communicates with exhaust fans or dryer exhaust, make-up air must also be provided so that the negative pressure created by the exhaust does not steal air meant for combustion. Designs that put the laundry (including a dryer) in the mechanical room should be avoided.