Ducking under/Autoland distance
Ducking under is going under the Glide Slope / VASI to hit the ground asap. 411A is right You might be able to do that on a Baby-Boeing but you will get cought if you try to cheat in a B747, B767 or B777. Same for the Busses: 320 You might walk away with it but A330 + A340 you can scratch the Undercarriage into the Metal before the concrete.
Our QRH, PI section, Company tailored, states + 2.500 feet required Landing distance for Autoland when You calculate the "numbers" for the Landing.
I hope that helped you. For more details required ask the manufacturer.
Fly safe and land happy
NG