Right on. You have three options. You can do the half-extract addition as suggested above in order to boost hop utilization as if it were a full boil. You can also just increase your bittering hop addition by about 20-25% to compensate for the lower utilization. So, if they want a 1oz, make it 1.25oz. Last you can boil for a little longer. Instead of 60 minutes, go with 75. I'd recommend doing a 2.5 gallon boil and only adding half the extract initially. Stick with the recipe for hop additions, add the second half of the extract with 10 minutes to go. You get the additional benefit of less carmelization which means you'll hit the target SRM more closely.