Leg and head hits use formula 1 and all other hits use formula 2.
Formula 1: (rand(0.0, 1.0) < min(max(item_weight * 0.33, 1.0), 2.0) * min(max((raw_damage - 40.0) * 0.5, 5.0), 15.0) * 0.015)
Formula 2: (rand(0.0, 1.0) + .05 < min(item_weight * 0.33, 2.0) * min((raw_damage - 40.0) * 0.2, 5.0) * 0.015)
If the random number generated is less than the result of the right side of the formula, then you get a knockdown.