Authoring content for use as Mesh Decal geometry is in line with any model creation. Since this geometry will not rely on projection (like a Deferred Decal Actor), the geometry
needs only to stay in front of the surface you want to affect. With this in mind, the decal geometry should "hug" the underlying surface, but it does not have to match it very closely
because there is already a small depth bias applied to account for this small offset. Also, using a bit of geometry that feathers out from the decal can be good to create the soft transitions
for the decal, which is something you cannot attain with a masked material.
Base Mesh and Decal Geometry (Isolated)
Another thing to keep in mind when developing your content, is that Mesh Decals can be a challenge with Levels of Detail (LOD), or long view distances when there is limited depth buffer
precision, because geometry will intersect (or not match-up) as intended. However, you can either change the mesh to account for this (in most cases), or you can use World Position Offset in
your material to adjust the offset without having to return to your modeling application.