Automatic landscape layers filling

i’ve read this tutorial Grass Quick Start | Unreal Engine Documentation , and i have a question: i’ve already made a material for my landscape that uses vertex normal to choose the right texture (grass or rock), and now i want to make layers like in that tutorial (grass, rock), and an extra layer “none” that will automatically choose the right layer (grass or rock) by the vertex normal

how can i make this “none” layer? or something to get the same result - automatically choose the right layer for the regions without any layer to fill 'em with grass and trees. and i wanna try to spawn rocks on the rock layer by the same way as grass is spawned in that tutorial

this is my current landscape material (without layers):