I think the best way to have that 100% line between your materials in a specific place you want is to split that part of your geometry so you can give it another material....you can do that in many ways using different softwares

hi everybody im using a HDRI environment on my product and i really like the result i just want to make the hdri a little bit blurry so the reflect doesnt show the details of the hdri map.......i used the blur scale and blur offset on the hdri window and the preview of the hdri got very blurry but ...