Guided Reading: Use this once a week as a formative assessment to see how students' fluency is progressing. Once you identify where students have deficits, make a goal for that students and work with him or her on that goal throughout the following week before using the rubric to analyze his or her reading again.
Reading Conferences: During reading conferences, sample students' oral reading of their current independent reading book or literature circle book. This will be a great indication to you if the book is too easy, just right, or challenging for the student. It's also a great conversation starter to talk with students about their independent reading material choices. In middle school, I feel like some students "fake read" and would rather pick a book that's way too hard for them rather than pick a book that they can access that's appropriate for their reading level and background knowledge. Using this rubric will help you quickly identify if a student is able to access their selected book at a "just right" level.
Summative Assessment: If you are a middle school teacher who does not get very much one-on-one or small group time with your students, perhaps you could use this rubric as a snapshot in time to see where students are at with their oral reading before Parent Teacher Conferences or the end of a quarter.
I hope that the rubric helps you identify goals for your students and also helps to monitor oral reading progress over time. Enjoy!