Custom nether structure generates above the bedrock

Hi !
I'm trying to generate a custom structure in the nether, but it currently generates above the upper bedrock layer.
How can I control the height at which it spawns while letting it generate anywhere between the sea level and the upmost bedrock layer ?
Here's my current structure.json:
{
  "type": "minecraft:jigsaw",
  "biomes": "#letsdoaddon-structures:has_structure/crimson_vinery",
  "spawn_overrides": {},
  "start_pool": "letsdoaddon-structures:crimson_vinery",
  "size": 3,

  "project_start_to_heightmap": "WORLD_SURFACE_WG",
  "step": "surface_structures",
  "terrain_adaptation": "beard_thin",
  "start_height": {
    "absolute": 0
  },

  "max_distance_from_center": 80,
  "use_expansion_hack": false
}

Thanks !