You determine your Rating of Perceived Exertion (RPE) by listening to your body during and immediately after the workout. The RPE-score should reflect how intense this workout felt to you on the day. That is why it is important that you give your RPE-score as soon as possible after a training session because it is a subjective measure and you should not allow yourself to put a ride in perspective. Furthermore, it is mostly about intensity and not about duration. A long ride at a low intensity can be pretty tough, but has a low intensity and therefore should have a low RPE-score.
JOIN will predict a RPE score for you, based on the overall intensity and stress levels from your ride. RPE is still a score you have to give yourself and will make the planning and analyzing of your JOIN workouts more accurate. If your RPE score is much different from the one that JOIN would predict, we take that into consideration as it might say something about your form or fitness. Our advice is to consider the entire activity, from the start to end, and how you felt at the end.
What we like to do is to mentally recap a ride in the last 5 minutes of it. Think about how it felt during the intervals and then how you felt at the end. If the intervals were very hard and could not be completed then the RPE should be very high. If they felt good and you could complete the intervals/ride with no real difficulty, then a lower RPE would be appropriate.