I understand industry standard for rods is 15mm and 60mm separation. My question, how far below the center of the lens should they be? Is there a standard?
Yes there is an industry "standard" that many manufacturers are following. I do not have the number memorized and am home so I cannot tell you the lens centerline to rod centerline height.
My recollection is that it is derived from the Arriflex lightweight support system made for the Arri 16SR and then adopted by Alfred Chrosziel when he adapted his film matteboxes to video cameras many years ago. This allowed the use of followfocus and other accessories made for that system to be used without modification.