Approximately 45,000 to 55,000 square feet of Usable Area. During the term of the lease, DCAMM reserves the right, if deemed to be in the best interest of the Commonwealth, to lease additional space, on terms and conditions agreeable to the landlord in the selected building should space become available. For the purpose of this RFP, "Usable Area" means, with respect to the Premises or any space removed from or added to the Premises, the square footage determined by measuring the entire floor area of the Premises (or such other space) bounded by a line established by the predominant inside finish of the permanent outside Building walls that abuts the floor (not from the inside face of the windows) and by the interior surface of corridor walls or other demising walls. Deductions are not made for columns or other structural elements, or for partitions subdividing the Premises. Notwithstanding the foregoing, under no circumstances does the Usable Area include major vertical penetrations such as ventilation shafts, elevator shafts, stairwells, atria, or lightwells, and their respective enclosing walls, and it does not include vestibules, elevator-machine rooms, and other building-equipment areas, janitorial, electrical, and mechanical closets, loading platforms, restrooms, and their respective enclosing walls, irrespective of whether Tenant occupies a portion of the floor, an entire floor, or an entire Building.