Desk Top - preventing sagging and cupping

I'm looking at a 72"x30" desk top and square metal legs.

1) Should the metal legs be at the end, or inset some amount (4-12"), or does it matter in terms of stability? 2) If I'm looking at stiffeners for the length to prevent sagging, should they be 60" or 48"? How close to the edges should they be (I'm not looking at aprons, I don't like hitting the apron with either my legs or the arms of my desk chair.) 3) Do I need stiffeners closer to the middle to prevent cupping? Would just a couple work at the 1/3s of the length work?