The boiling times for the hops are the same, but you reduce the amount of hops needed due to better utilization by boiling a lower gravity wort.
The easiest way to figure it out is to use some brewing software, like Beersmith. Put in the regular recipe and see what the IBUs come to. Then just switch it (by changing equipment at the top of the page) and see those IBUs. Adjust the bittering addition down until they match. I hope this makes sense- it's really super easy to do in Beersmith, but tough to explain how to do it. Otherwise, I don't think it's a strict "20%" reduction thing, but that might be close.