There are some mixed ponts here.
K is not for necessary Key, is for blaK. Key could refer to other things in a photo. (There was a thendency to say K is key, meaning that how you use black is important, but in my humble opinion that is old fashion, for example there was a chromathic conversion and achromathic one, etc.)
The maximum amount of ink TAC is determined by the print specification you are using, for example in USA you commonly use SWOP, Gracol or Fogra. In Europe you could use Eurocoated of Fogra.
Aditionally there are some sub sets depending on the specific type of paper used. Mainly coated or uncoated, but it could be some more specifics yet, like newspaper on a rotary press.
This kind of values:
Black - 90%/95%"
are when you are trying to make a patch of black print, but you can not have a "generic" value, without considering the specification used.
For example Let us asume you are printing a photo of the deep space, so you have a pure RGB black.
If you convert the photo in Photoshop using a North america SWOP 2 coated profile, the result will give you a max ink of 300% using c75m68y67k90.
That is the "deepest" black the profile allows.
Using a Fogra you have a 330% max ink.
But on an uncoated paper you will have less percentage, for example arround 220%.
You simply use the eyedroper tool and read the deepest black on your image. But once you methodology is well established using profiles you never measure that again.
The white part is due a specific fenomenom of ink viscosity and ink absorption on the paper.
You can not shoot or print only a few molecules of ink. As it is a fluid you can print a droplet of some minimum size.
Add the fact that some of the ink will run thru the paper fibers due capilarity. (Dot gain)
So if you have a gradient of 0% to anything, you see a step from pure white to some initial color.
That is why some inkjet based printers has light cyan and light magenta.
That minimum 3%-5% normally is used on uncoated papers where this paper caplilarity is greater.
But now days you do not manipulate CMYK curves, you simply asign proper color profiles or configure that in the proper Photoshop panel.