For the n+1 cart, it was always the smallest diff between n and any remaining carts.

Picking the first n, was more difficult. A strong hint was if the ROM name contained a variant of the word Japan. Otherwise, imagine you had 4 images, you could diff them 16 ways, but half of those ways are mirror images of each other ie. a diff of ROM 2->4 is the inverse of of a diff of ROM 4->2.

So ideally, the first ancestor would be the image with the most amount of the smallest children.