I find making it entirely metal, entirely rough, and black diffuse works best. If it's 0% metal, you'll still get a little specular. But setting it to metal will use the black diffuse as the specular, and it'll just be a void.
first of all im sorry maybe i sound like an idiot asking this im quite new on this PBR thingy and just wondering how do most people bake a fake hole on their low poly ?? i mean albedo/diffuse map shouldn't contain any shadow information ,right ?