here is an example of how you might use the TargetLengthBasedClassNameAbbreviator class in a logback configuration file:

<configuration>
  <conversionRule conversionWord="abbreviatedClassName" converterClass="ch.qos.logback.classic.pattern.Abbreviator" >
    <abbreviationClass>ch.qos.logback.classic.pattern.TargetLengthBasedClassNameAbbreviator</abbreviationClass>
    <length>20</length>
  </conversionRule>

  <appender name="CONSOLE" class="ch.qos.logback.core.ConsoleAppender">
    <encoder>
      <pattern>%d{HH:mm:ss.SSS} [%thread] %-5level %abbreviatedClassName - %msg%n</pattern>
    </encoder>
  </appender>

  <root level="debug">
    <appender-ref ref="CONSOLE" />
  </root>
</configuration>

This configuration file sets up a CONSOLE appender that outputs log messages in the format "%d{HH:mm:ss.SSS} [%thread] %-5level %abbreviatedClassName - %msg%n". The %abbreviatedClassName part of the pattern is where the abbreviated class name is inserted into the log message.

The TargetLengthBasedClassNameAbbreviator class is used to abbreviate fully qualified class names, in this example it's abbreviating the classname to 20 characters. By setting the length to 20, it abbreviates class names to their last 20 characters.

Please note that, the output will be written to the console in this example, you can use file based appender to output the logs to a file. And you can refer to the logback documentation for more information and options available.