The 50% utilization is totally wrong, it never gets that high, max is closer to 35%, and utilization is definitely not linear with time or temperature either. It isn’t far from a square root relationship vs. time, however. I’ve developed the following equation which takes all these things into consideration in a very simplified format. This is an approximation that will come within 5-10 IBUs of Tinseth. Try it out and see.

IBU = oz * AA% * [sqrt(5*Boiltime)/V + sqrt(2*HStime)/V]

where

V is post-boil volume (in gallons),

Boiltime is in minutes, and

HStime is post-boil hop stand time in minutes where it hangs around between about 150-190 F (65-90C).

(Sorry for my Yankeeism, my equation uses ounces and gallons. If you like grams, then divide by 28.35. Liters, multiply by 3.79.)

For multiple hop additions, put each one in separately, run the calc, then add up the total IBUs. It's not perfect, but it's pretty darn close for standard gravity worts of 1.050-1.060. At higher gravity (e.g., >1.080), change the sqrt 5 to 4 instead, and sqrt 2 to 1.5. At low gravity (e.g., 1.035-1.040), change the 5 to a 6, and the 2 to 2.5. Pretty close, and not too hard to memorize.

Keep in mind also, that regardless of amount of hops used, you'll never get more than 90 to 100 IBUs by conventional methods. It is simply a fact of physical chemistry. If you use more hops than you need for that IBU level, you just waste hops and beer (the extra hops permanently soak up extra wort or beer).