Top-born children reach the top with odds 3-to-1; bottom-born children with odds 1-to-4. Set the slider to the odds ratio $OR = 3 \div 0.25$.

12 (answers within ±1 count)

$OR = 3/0.25 = 12$: birth at the top multiplies the odds of ending there twelve-fold. An OR of 1 would mean origins are irrelevant, this society is a long way from that.