Adding a fruit puree at flame out or whirlpool and it's effect on IBU

I am formulating a new fruit beer recipe and I'm planning on using a technique gotten from a professional brewery. They make a very simple wheat/fruit beer but they add seedless fruit puree at whirlpool. The puree is between 13 - 15 brix which means it is about 1.057 specific gravity.

Given that I will mash the grains and end up with a wort that is 1.045 without the puree and then add some bittering hops during the boil, how do I calculate the effect of the fruit puree's gravity on the hops? Do I calculate the bittering for just the wort when I add the hops or do I calculate it for the wort after the hops have been added and boiled for an hour and then the puree is added?

IBU contributions only come into play when the hops are utilized during the boil. Since you're adding to the gravity after the boil is over, you shouldn't need to factor the puree into the IBU calculations. Just calculate based on your base wort gravity.

I thought that would be the case. So if I want 15 ibus I will add based on the sweet wort from the mash, ignoring what will be added later post boil.

Thanks!