I've followed the formula, but....
Nothing is more frustrating than when you have taken the time and been extra careful to weigh out and mix a PMS blend and then the color looks nothing like the book! You have cleaned your knives between dipping out the colors (no cross contamination in your base inks!) and mixed it until there are no streaks of unmixed inks. Double-checked the formula to make sure you didn't accidentally move the decimal, everything! And still the color looks a little off, or worse a lot off. It looks like it needs yellow, or black, or something else that's not even in the formula. So you throw it away and start over and when you check the color again..same thing!
Noone wants to think that the formula could be wrong, but sometimes inks just have to be tweaked. This is when it is important to remember that the best judge of color is your eye. If it looks like it needs a little green even if there's none in the formula; add a little green. Trust your gut and make the add, start in small increments and you should be able to tell in a few adds if it's moving the way you need it to. Experience is golden here, and the only way to get it is to just go for it. You'll probably end up making some mistakes and having to start completely over a time or three, but that's just part of the cost of fine tuning your eye, it gets better as you go.
Of course, if you get too frustrated or think; I'd rather be printing than spending all this time messing with ink, jump onto the store and navigate to the drop down menu under shop to "ink", go to the second page (4th item down) and find the item marked "Mixed PMS colors (blends) and just order it. We got you covered