Soooo, you want the Terrain to be Exactly Flat at 32?
If you want that, its a bit tricky. Try a bit with thees Settings in your BiomeConfig:
You could also try tweaking these in your worldConfig a bit into the Negative:
But, to be Honest, if you want Flatlands, you can do that without TerrainControl though, would be much easier.
If your ReplacedBlocks solution is what you desire, you can use these Replace Settings:
Or if you want the the Default 2 Dirt Layer then Stone: