Znalazłem odpowiedź w tym artykule.
http://byo.com/compo...lume-techniques
Dokładnie w tym fragmencie tekstu:
To completely master high-gravity brewing calculations, all you need to know is one simple formula:
C1V1 = C2V2
where C is concentration and V is volume. The subscripts refer to the initial strong beer and resulting blended beer.
For calculating gravity, use “gravity points” — the decimal portion of specific gravity as an integer. For example, a specific gravity of 1.045 equals 45 “gravity points.” If you had 5 gallons (19 L) of wort at a specific gravity of 1.064, what would the gravity become if you diluted it to 7 gallons (26 L)? Substituting the values into the equation, we get 64(5) = X(7). Solving for X, we get 64(5)/7 = 45.7, or a specific gravity of almost 1.046. You can also use degrees Plato for these calculations.
You can use IBUs and SRM values as a concentration © in these equations for calculating bitterness and color values. For example, if your 5-gallon (19-L) batch of beer had 50 IBUs, the equation 50(5) = X (7) would give you the resulting IBUs in your 7-gallon (26-L) final beer. (It would be 35.7 IBUs.)
Using brewing software, such as ProMash, you can calculate your OG, IBUs and SRM for your strong beer. Then, you can calculate the values for the diluted beer using the “CV” formula. This way, the influence of wort gravity on bitterness will be taken into consideration.
Dla mojego przykładu (i IBU 30) rozcieńczenie brzeczki 2,75 litrami wody (z 13,5°Blg do 12°Blg) spowoduje zmniejszenie IBU do wartości 26,6.