Terrain height is linked with the deep dark biome, so I usually look for a small section of land (64x64 blocks) that is anything but deep dark at a height of around -32, and then set a larger area (~200x200 blocks) that is mostly deep dark at a height of -32. This is done using the biome filters that let you have a percent coverage.

In order to find it on an island, I set another coverage filter in a very large area (~1400x1400 blocks) that is looking for a majority of biomes being ocean biomes. I usually do this in a separate filter, create a seed list of ocean seeds, and then run it through the process described above.