Hi Paul,
its quite easy to calculate: 1 IBU = 1mg of isomerized alpha acid in 1 Liter of wort.
For example: your batch size is 25l and youd wish to have 25IBU in it.
25mg isomerized AA * 25 Liter = 625mg isomerized AA.
If youd like to dilute the wort, just use the rule of three.
Cheers :icon_cheers: