Object-oriented design principles are fundamental concepts that carry important design knowledge and foster the development of software-intensive systems with a focus on good design quality. They emerged after the first steps in the field of object-oriented programming and the recognition of best practices in using this programming paradigm to build maintainable software. Although design principles are known by software developers, it is difficult to apply them in practice without concrete rules to follow. We recognized this gap and systematically derived design best practices for a number of design principles and provide tool support for automatic measurement of these practices. The aim of this paper is to examine the relationship between design best practices and 10 selected design principles. This should provide evidence whether the key design aspects of the design principles are covered. We conducted focus group research with six focus groups and 31 participants in total. In parallel, each group discussed five design principles and assessed the coverage by using the Delphi method. Despite suggestions of additional design practices that were added by the participants, the result reveals the impact of each design best practice to the design principle and shows that the main design aspects of the design principles are covered by our approach and is therefore feasible to derive concrete design improvement actions.