Needs your help for hacking IBU of a fresh wort beer kit

It's a 15L fresh wort beer kit, which normally needs 8L of water addition. It is supposed to comes at 1.044 / 20 IBU with the water addition. If I don't add the 8L, brewtoad tell me it's 1.067 and 29 IBU.

Now 1.067 is perfect. But I would like to take the beer to about 40 IBU.

SInce I have spare DME on hands I could use, I would like to do a 1 litter boil of a 1.067 DME/water mix for 15 min with hops added at boil, which after being added to the 15L fresh wort would hopburst my beer to 40 IBU.

So my question is.. finally :

How much IBU this 1L addition should have to kick the beer to 40 IBU?

I haven't found a proper dilution calc for that yet. Does any science-beer geeks can point the math solution to me ?

1. You're gonna wanna boil those hops for an hour. 15 minutes isn't enough time to really isomerize those alpha acids.

2. You might as well just pretend IBU's dilute linearly. They probably don't, but you'll probably introduce more error from the inherently unscientific process of brewing in your kitchen than you will using a bad formula.

3. If you find yourself wanting close to/more than 100 IBU's in the DME wort, you'll probably wanna go with more volume; a liter of wort can only absorb so much hops before isomerization efficiency starts to suffer.

Thanks feinbera for the tips there!

I am still looking for the IBU I need in the wort to make it 40 IBU in the beer.

Listening to feinbera I changed the addition volume for 4L instead of 1L.

Heres the math's according to a dilution formula I found by googling. We are looking for X, which will be the IBU of the 4 liters addition :


29 IBU * 15 L = 435
x IBU * 4L = 4X
(435 + 4x) / 19 = 40 IBU
435 + 4x = 760
4x = 325
x = 81,25 IBU

Yeah!

Almost ready to roll. What do you guys think of the whole idea (4L of 81 IBU wort addition to the original wort) ?