Just use percentages: A counterbalance truck uses a 12' aisle, a reach truck uses a 9' aisle, and VNA truck uses a 5-6' aisle depending on style. Raymond, Crown, and Jungheinrich all make very good swing mast style VNA trucks. Landoll makes the Drexel which can operate in roughly a 56" aisle with a standard load. You can see that it's apprx. 25% more storage for each category that moves closer to VNA. Be sure to maximize your vertical cube as well !