Since the FG is what measures the residual sugar in the final beer, why do we use OG, instead of FG, to calculate the ratio of the bitterness over the sweetness in final beer? How precisely is the quantity IBU/GU?
This is a good question, and I've talked to a few people that agree. I think it's just the nature of the recipe definition/creation process (especially historically): we control most directly the OG, not the FG, even if we're able to anticipate/estimate it. But, yes, we're really trying to control the bitterness:sweetness ratio in the consumed beer, and FG is our proxy for sweetness.