How to complettly override height channel produced normals with a new layer normal map?

Without any visual step  on mask edges?    If  I want to put a material  on the  top that would use only its own  normal map and override/replace  any  height produced  normals  of layers  beneath?   Is there a blending trick for that?       I try to use flat height and replace blending for heigth of this new layer    but it creates a visiable step  around  mask edges.