We precisely measure extract, grain, fermentables, hops, and yeast for our wort/beer because the slightest difference will have an effect on gravity, sweetness, IBUs, alcohol presence, etc.
The same mindset does not relate to spice additions. These affect flavor and flavor alone. And you cannot gauge flavor by following a recipe and not tasting it. I would never trust a recipe that tells me to add 2 tbsp. of salt to my cold pasta salad. I would use common sense and taste, adding a little bit at a time until it's just right.
That being said, I recommend that you make about a cup of concentrated spice tea on the side with boiling water (or wort) and whole, cracked spices. Taste it when done. It should be quite intense/potent; you won't be able to chug it down, but it shouldn't taste like ass either.
Strain out the solids and add this liquid spice tea to your wort at flameout, tasting all the while to be careful not to overdo it. If more is needed by the time you rack to secondary, then add more of the spice tea. It's really as simple as that... I'm not sure why this old idea hasn't caught on yet.