[dzlater]

I am starting to think about what to brew at our clubs Big Brew Day.
Last year with so many people brewing, trying to chill was a challenge with the lack of water pressure for my chiller.
I was considering doing a partial boil and topping off with cold water.
I can figure out for example how many ibu's I'd get with a one hour boil for a final volume of 3 gallons @ 1.080
Then top off with 3 gallons of cold water to get six gallons @ 1.040.
If I calc the ibu's of the 3 gallons at 80 does that mean the ibu after the top off water is 40?

[malzig]

Yes.  3 gallons of 80 IBU beer will make 6 gallons of 80x3÷6= 40 IBU beer, diluted.

The only thing you need to watch out for is the maximum IBU's.  Theoretically, that's 110, but Russian River says they've never been able to get past ~90 IBU, no matter what they tried.  There have been some other studies that have topped off as low as the mid-50s, but that seems unusually low.