If the recipe calls for 1 oz of hops @ 8% (1 x 8 = 8 BUs) I would substitute 1/2 oz of 16% or 2 oz of 4% (.5 x 16 or 2 x 4 = the same 8 BUs). Generally, if the AA% of the hops I have is within 10% of the % called for in the recipe, I ignore the difference, as long as my recipe still falls within category guidelines. More @Wikipedia
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