When you’re checking out a linearization target measurement in a 3D program like ColorThink, the different ramps of color look like the legs of a spider.
youtube.com/watch?v=tXQUPe29 … e=youtu.be
As you say you can visually see where to cut off your ink limiting with the different colors, but the black ramp as it heads darker is not quite so obvious. For some measurements, if you look closely, you may see where the black actually gets lighter with more ink. This can be due to bronzing or pooling of ink when there is an excess. You’d have to look closely and follow each ink patch (like I demonstrate in the video above); you can’t just look at a spider graph and assume that each point is laid out in order to the darkest. Obviously, you’d want to cut off your ink where you get the darkest point, and before it starts to get lighter. On some systems, the black line start to curve like a corkscrew around the neutral axis. To have a stable printing workflow, you would want to cut off the black ink before it screws around like that.
These two scenarios are not too common nowadays. If you have a well behaved inkset and printer and it just steps down to the darkest black it can hit, then call it good. The viewing of linearization ramps in a 3D graphers is perhaps more useful for the colored ramps than for black. It all depends on what your purposes are too. If you want the most gamut, you can allow more ink. If you instead want a press that has more stable color or want to save ink, you can limit the ink more.
Regarding your second question,
There will be a black patch made up of 100% K ink only, and there are black patches made up of CMYRGB and K ink. These rich black patches will generally be darker (have more density). It’s hard to say "never’ but I don’t think I’ve ever seen a system where the CMYRGB without K ink patch was darker than 100% K.