No, you haven't wasted my time at all. If I didn't have time to spare I wouldn't be on here and so I'm happy to help.
Okay, so I've made the decal you requested - that shape in solid black at 60% opacity - and, having tested it on a chrome Gr.2 GT-R I fear its not going to be quite what you were expecting.
The problem is the decal doesn't make the chrome look darker but actually makes it look like grey paint with a layer of clear-coat over it. It completely removes the chrome effect.
Similarly it also doesn't work with "candy" paint either. For every other special colour (fluorescents, flakes, matte, metallic, etc) it does exactly what you'd expect (make the underlying colour appear darker) but chrome and candy paint are a total no-go. Whether this is physically accurate (since GTS is supposed to use Physically Based Rendering techniques) or whether this is a error in GTS's shader engine I could not possibly comment, but what I do know is that it doesn't work (unless you actually want the roof to be hell-hole-prison-cell grey?)
For what its worth I still think that the darkest chrome (K1), on its own, with the just the decal logos place on it will get you 99.9% of where you want to be for the roof of the 2016 Craft Sport car. The light fringes around logos/shapes when on chrome or candy is a bit of a disappointment but that is also directly related to the same alpha transparency issues we have with transparent gradients.
