I have tested further with the following result. I have 2,25 kg Pilsner malt and 0,50 kg Wienermalt and 30g of Northern Brewer, 9,9%.
If batch-size is 9 liters I get IBU 71,3. If I increase the batch-size to 10 liters the IBU calculated is 68,2. When I set the batch-size to 11 liters the IBU is 0. With 12 liters IBU 2,6, 13 liters 10,6, 14 liters 18,3. With the batch-size 20 liters the IBU is 33,8!
Confusing.
//Goran