Ideally with the Physical material you'd approach things like you would in a physical sense. Is the material a non-oxidized pure metal material? Go with metal. Is it a painted metal? Then paint is a non-metal material applied on top of a metal material and so you're basically recreating just that top non-metal paint layer.
If you've got paint on top of a metal material but the paint is sort of incompletely applied and you can see the actual underlying metal? Using a black and white mask as a metalness map could be the way to go.
Alternatively (and I prefer this workflow myself) you can also use the layered material, put the metal material as a base and a painted material on top then just mask everything with a nice punchy black and white mask. I prefer this approach because you're thinking about layers, just like you would in the real world and you have easier control over both materials. It might be a bit slower to render though.
To control the reflection strength of your metal materials you can play with the base color value and if you're into them details then tweaking the edge color is also important. In both cases I'd try to compare my material to the real world reference as I'm tuning things.
One quick note, technically using values between 0% and 100% for the metalness is physically incorrect. If you're using in between values you're basically just blending between a metal and a non-metal material which... Is not physical. Can be useful though so that's why we've left it in. Also, if you're using maps for the metalness make sure they're also either fully white or fully black because again, in between values... Not so physical :)
Not sure if the above helps but hey, that's why we've got the forums here to discuss things.