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 /usr/local/share/qbrew/data. 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.
QBrew has many command line options. If you type qbrew -help in a terminal, you will get a list of all available command line options for QBrew. Probably the most useful is to pass the name of a recipe to qbrew, as in the following example: qbrew mybestale.qbrew.