Hop utilization with top up water

Hi folks. Quick question. I'll be brewing 12 gallons of finished beer and then topping up with 3 gallons of water in the fermenters. I have set up an equipment profile to account for the top up water and beersmith has adjusted the grains and hops. I'm wondering about calculating the extra hops to add due to the lower utilization in the higher gravity wort.

Would it be proportional to the percentage of water I'm adding at the end(25%)?

BeerSmith is calculating IBU based on the batch size (amount in the fermenter). Since this includes the top up water, the IBUs are calculated already.

Hop utilization isn't linear. It happens fastest in the first 20 minutes of wort contact time and then slows. The only thing I'd be concerned with is whether the total IBUs in the undiluted wort exceed 100 to 110. Immediately post boil, IBUs can be higher than 100, but quickly fall to about 100 as isomers either break or bind to proteins and polyphenols. So, if possible, you'll want to top up in the kettle, before you chill.

Thanks! Great answer on a forum for once. My plan is definitely to top up in the kettle before cooling.