You can calculate the actual alcohol percentage in a batch by using the Alcohol Percentage tool unser the menu. Enter in your measured original gravity and final gravity. The percentage of alchohol by volume and by weight will be calculated.
You can access the Hydrometer Correction tool under the menu. This tool is useful for converting your actual hydrometer readings to the correct value. Enter in the temperature at which you took the sample, the temperature at which the hydrometer is calibrated (typically 60° or 68° Fahrenheit), and the actual hydrometer reading. The corrected reading will be calculated.
The data that QBrew uses is in the file qbrewdata, typically installed under the install directory in Windows, as part of the application bundle under Macintosh OSX, and under /usr/local/share/qbrew under traditional Unix.
If there exists a file in the user's home directory with the name of .qbrewdata, then this file will be used instead. It is simple to add new ingredients, or modify the values for existing ingredients. Simply copy over the main qbrewdata file to the user's home directory, rename it to .qbrewdata, then edit it with any text editor. The file format is in XML and should be self explanatory.