Isomerization is non-linear: Utilization
You get more bittering per unit time early in the boil than later. The best way to control your bittering is to calculate based on AA % rather than weight and add a efficiency factor, which can only be learned by experience. Try aiming for 85% of the bittering on your next batch and see how that works. Since bittering utilization changes more slowly the longer you boil, trying to control the IBU by timing is tough. Adjusting the amount of hops is much easier.
I suspect the non-lineararity is due to the smaller grains of resin being consumed early in the boil.
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