This paper presents a comprehensive approach to enhance the traditionally deterministic-based power grid planning using system well-being analysis concept. The objective is to identify and characterize the existing system reliability concerns inherited from the adopted deterministic criteria, so that power utilities can accordingly adjust their reliability criteria to cope with real-life system uncertainties and hence to enhance the overall system reliability. A determination of transmission capacity reserve derived from incorporating deterministic criteria into a probabilistic framework has been introduced in the paper. An application of the proposed methodology for justifying a transmission addition project to accommodate the system load growth is illustrated by using an actual island power grid. Both technical and economic aspects, greatly concerned in practice, have been taken into consideration in the project justification.