In this work, we propose a methodology to measure the performance and usability of virtual, augmented, and extended reality (VR/AR and XR) devices, providing a number score instead of traditional questionnaire-based assessments. In order to accomplish this goal, we have designed a target selection test in immersive environment, and we carried out a mathematical study to measure the test quality. We defined three quality metrics that provided a score within a scale from 0 to 10. The idea is that, instead of assessments based on questionnaires which are filled out by the users in the end of an immersive test, the quality experience can be provided by a number with no bias, taking into account the factors relevant for that application. Then we validate our methodology by performing a user test with Meta Quest 2 with ten participants. The proposed methodology can be used to compare VR/AR/XR devices without the subjectivity of questionnaires.