I dont claim to know what the industry standard is but I would say you'd shoot for the middle. Think in terms of maximizing the power density of the beam -- smaller beam is always better. The beam comes through the focusing lens and forms an hour glass shape. The most power is at the cross-section of the beam waist -- the area of highest power density. I think you want that beam waist to be at the middle of the part, because that makes the beam size at top and bottom of the part at their minimum diameter.