I'm over here using the real ibu formula like... Huh?!?!..
W= B ∗ V
1000 ∗ A ∗ U
Where W is the total weight of the hops to be used in grams, B is the desired
bitterness in IBUs, V is the final volume of post boil wort in the kettle in liters,
A is the alpha acid content of the hops (4.7% AA means that A = 0.047), and
U is the assumed hop utilization.
For example targeting 16 IBUs and assuming a utilization of 28% (i.e.
set U = 0.28 and B = 16). Utilization will vary from system to system; if your
beer ends up too bitter, try raising the utilization to 30%, and if it ends up
not bitter enough try lowering it to 25%. For example, assuming your post boil
volume is 21 liters, your hops contain 4% alpha acids, and your utilization is
28%, you would use a total of 30 grams of hops.