Asigning a spot color on a document has the purpose that anyone in a system can identify and apply a specific color on your printed project.
But if you have control on this process and you know what are you doing you can brake some rules.
For example, In my opinion the formula guide lacks of a pretty red.
The closest to a c0m100Y100k0 red are warm red and the 032 red but they are too orange for my taste.
So I can simply asign a 032 red on my file, elaborate the plates and in the printing facility I ask a modification of the color.
This modifications can be done either "by hand" for small projects on site, or you can ask for a specific formula and you get lets say 10kg of ink that you can store for the rest of the year for a big project.
Formula means you have the proportions of the base colors. 10 parts of "banana yellow" plus 3 parts of "cherry red".
If you have this recipy you can make another 10kg for another day.
You can also record the raw color values with a spectometer to asure the same color betwen diferent lots.
I have played with this process playing with some metalic colors.
By hand I mean for example, the P116c is a yellow with a bit of warm red (97%-3%)
If I want to keep the same base inks, the next yellow is P1235c (90.6%-9.4%).
But the diference is too broad. If I want a color in between I can try lets say starting with the 116 and adding a bit more of warm red (95%-5% for example).
If you are making lets say 1kg of ink you can simply mesure the parts. 950g of one and 50g of the other.
Sometimes you need to prepare just a small amount of ink (half a cup of coffe) For example when adding black just the tip of a pencil can make a very large diference. So you add it and mix it. Add more if needed.
As the inks are transparent, it is difficult to judge the printed color. A print specialist can play with a drop of ink and smudge it on a paper to "match" the printed color.
So when I say by hand... literaly is by hand.