It's about finding the right hip angle. KOPS can be a decent first guess, but is otherwise meaningless. You find the right hip angle for yourself and maintain it. On an MTB, you'll be way up in front, so the saddle needs to go back to rotate your hips (centered about the bb spindle) and maintain the angle. On a TT bike, you're super low, so it needs to come forward. All the positions in between... are in between.