Comparative figure for bitterness are the International Bittering Units, defined as 1 IBU = 1 milligram of iso-alpha-acid per liter. There are different calculation models and tables available for estimating this value. I'm using the formulas supplied by Glenn Tinseth but take a higher utilization for hop pellets into account (I like to muck around). With the Java-Script simulation below (works offline!) you may observe, how bittering is affected by variating different parameters of 3 hoppings. Calculating the required weight of hop for given IBU-targets is also possible.

The composition of a brewing water has a big influence on the quality of your brew. Brewing water chemistry is rather complex and unfortunately the deep insight given in many brewing texts is very complicated and confusing. But that needn't be so. I'm water chemist and could make big efforts but I've learned that an easy approach will do to adjust almost all waters to any beerstyle.

Let's focus on the essential elements first. Depending on the underground formation the water arises from, it takes up more or less mineral salts. When this salts are dissolved, they form ions -- positive or negative charged elementar particles. Ions of primary concern for the brewer are Hydrogencarbonat (HCO3⁺), Calcium (Ca⁺⁺), Magnesium (Mg⁺⁺), Chlorid (Cl^-), Sulfat (SO4^--) and Hydrogen (H⁺, acidity -- given as pH-value).