Partial Mash IBU question

[Shepton]

Hi,
I am a little confused when trying to determine the IBU value for my partial mash recipe.
While Brewers' Friend provide a great on-line calculator it requires the target gravity OG to give an IBU.
With my partial mash I have a wort of 11ltr which has an approx gravity of 1.020. Im adding the bittering hops for a 60 minute boil - straight forward so far - but then I'm adding 1.6kg of DME at 45min - so for the last 15 min of the boil, the gravity will be very high (OG for the completed wort of 19ltr = 1.055) and consequently the attenuation (is this right?) will be less in those final 15min
Any ideas how I can accurately calculate the IBU?

 

[chub1]

There is a drop down on fermentables,(add custom)you can tick late additions and go from there

 

[Simonh82]

As Chub say, use the late addition option so that it is only counted for the last 10 minutes of the boil. I would also say that if you are adding the extract late in the boil it will make very little difference. Most of the extraction goes on in the first few minutes and levels off at about 60 minutes. At 45 minutes into the boil you will probably have got 90% of the bitterness that you would if boiled in low gravity wort for 60 minutes.

 

[Clint]

Funny how the calculator works in this respect. I do the ibu calc before adding the extract to the recipe,take note of what you need then add the extract to get your abv. This changes the ibu but you already know this now...