There are two general approaches to brewing – technical and creative. The technical brewer is more likely to take the time to plan the brewing day, refine the recipe, and take into account all the details that can affect the finished beer. The creative brewer is more likely to brew beer on a whim, adding a drop of this and a handful of that, often using ingredients that he has or grows in his yard. Personally, I belong to the first category. I like to play with the numbers in the brewing program, trying to get into the range defined by the style guides. There is something very satisfying about having all the numbers (density, bitterness, color, alcohol) fall within the style I want to brew. As a homebrewer, however, I’ve learned that balance is needed. It’s the balance that makes the beer drinkable and keeps the brewer sane.
Instead of precisely measuring hops to the gram, learn how to make other changes in the process. Personally, I buy hops by the ounce (28.4 grams) and try to have each hop addition counted in one ounce portions. If I find that the IBU is too high or too low, I move that addition closer to the beginning or end of the brew, but I don’t change the amount of hops. This greatly reduces the number of bags of hops in the freezer that have begun to accumulate and then have to be put to good use. The 28g portions are not always handy, but this is what I use most of the time. Sometimes half an ounce of high alpha hops does go back in the freezer, but I usually find a use for it the next time I brew.
Remember, calculating IBUs is like taking a shot in the dark. Hop condition, boiling strength, and three hundred other different variables affect the result. If you learn to lean toward creativity and not go into too much detail, you’ll sleep better and find balance in your brewing.