Tinseth Formula and Gravity

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.

It's an average value. Here is Glenn Tinseth's short write-up on his method:

http://www.realbeer.com/hops/research.html

Thanks. I was so close. I found this:
http://realbeer.com/hops/FAQ.html#units

which left out the part about the average volume.

Thanks for those links, guys!

I can't find it right now but there is at least one 'HBT science forum charter member' (for lack of a better term) that believes that wort gravity has little to no effect on hop utilization. And he makes a convincing argument for it. IIRC, it's more to do with the amount of break material which in many cases does correlate to wort gravity but not it's quite as simple as that.

EDIT: Found one thread on it.

Wow! Thanks for letting me know that.