How tall of a vehicle are you going to lift? Without knowing this, how does the lift manufacturer know how high the ceiling should be? If a lift, lifts to 6' and you put a double decker bus on it, do you think a 12' ceiling height will suffice? Have surface mount lights or a ceiling fan, you need even more height.
Using the ceiling height specified by the lift manufacturer for your ceiling height is like believing a shoemaker who sells you a pair of shoes that you won't have to duck for 6' doorways. How the frick can the shoemaker know this? Of course the lift maker will be able to specify a ceiling height if the lift is a 2 post with a top cross bar, but otherwise you need to add the maximum height the lift will lift to and then add the height (undercarriage to top for 2 posts, standing height for 4 posts) of the vehicle you want to make sure you can lift to full height. Pretty much the same way you figure out how tall you'll be in a pair of shoes.