Due to the continuous penetration of computer systems into our daily lives, their dependability is critical to the users. Particularly, ubiquitous embedded systems, such as elderly care and medication management, need to provide a high degree of dependability, since their failure may result in loss of a human life. Developers, users, and other stakeholders of such systems thus require mutual agreement on the degree of systems' dependability. In this paper, we propose to use an extended form of assurance cases, called D-Case, to describe how a system achieves dependability with meaningful evidences. One of the two major features of our scheme is to use a common tree structure in D-Case to enable developers and users of different systems to compare their dependability. The other is a scheme to calculate the degree of dependability from D-Case. Users can learn the degree more intuitively than grasping multiple metrics like MTBF and MTTF, or reading through a D-Case itself.