VoxeLibre

The Nether is 128 blocks tall, and Ancient Debris can be found between the bottom of this dimension and 119 blocks above bottom. So basically the last 10 layers don't have Ancient Debris but it exists everywhere else.

This ore is very rare, but the best chance (0.004%) to find it is between Y layers 8 - 22 above the bottom. Outside of this section of the Nether, below and above you get the same chance (0.003125%).

A less common approach that I like to use sometimes, is to use Fire Resistance potions (from piglin bartering) and dive into lava lakes to look for exposed Ancient Debris. Sometimes you can easily spot a few blocks, which is great.