ent_fixnormal is easy to use. just after you deform the mesh you call the function and you just plug in a pointer to that terrain and the frame number ( which for most terrains unles animated is usualy frame1)

looking at georges AUM 8 code I would place it in the deform function
Code:

---

function deform(vertex_number)
{
	vec_for_mesh(temp, terrain1, vertex_number);
	vec_scale (temp, 0.7); 
	vec_to_mesh (temp, terrain1, vertex_number);
        ent_fixnormals(my,my.frame);
}

---

it but should solve most NORMAL problems with the terrain