The Tinseth formula for hops utilization is
Decimal Alpha Acid Utilization = Bigness Factor * Boil Time Factor
The boil time factor is pretty clear. The bigness factor is
Bigness factor = 1.65 * 0.000125^(wort gravity - 1)
What is the wort gravity (e.g. pre-boil gravity, average boil gravity, some kind of integral)?
I'm playing around with recipator, and it looks like they use the pre-boil gravity, but that doesn't mean that it's right.