One site can not fulfill the requirements for all life stages. The overall HSI gives the most limited HSI for all four life stages. There are several different types of HSI methods that can be used. The Fish Calculator makes use of a method that only requires five factors. These factors primarily deal with stream water quantity or quality.
More complex methods employ many more specific factors.
Note that each input indicates a range of possible values. Notice the HSI values for overall and each life stage. This particular habitat is best suited to spawning and eggs and least suited for adults. Therefore, the overall suitability is the same as the adult suitability, the most limited of the four life stages. The two factors which indicate water quantity or stream flow are the stream velocity and stream depth.