Jeff,
The intent was to approximate the calculations you might see for an equivalent hop addition - it is calculated in much the same way any other hop addition including use of the boil size and current hop equation.
I'm not entirely happy with the current setup - as it is harder to use than it needs to be, and does not scale as well as the other ingredients.
I'm thinking for the future it might be easier to specify a simple measure such as a number of IBUs per several pounds for a target volume, and then scale by volume after that. For example one might specify 20 IBUs for a 5 gal batch as the contribution of 3.3 lbs of the hopped extract, which would scale up or down depending on the actual target volume.
The other nice thing about such a system is that it would probably be easy to estimate from the style what the approximate IBU content of the hopped extract is.
I would appreciate anyone else's thoughts on this - I searched for some time to find accurate manufacturer data for hopped extract but have not had much luck finding it.
Cheers!
Brad