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This is an excellent question (keep them coming) as it allows/reminds me to point out a few things and introduce some nuances/concepts. Firstly, I MIGHT use such an app, however, while it may sound counter-intuitive, I might not use it to stop a workout at the right time. Why? The first reason is that I can learn from analyzing why CD is below or above 10%. Also 5-10% is a range/rule of thumb so what number would you pick? Would you stop at 5%? 7%? 10%? 11%? I like to monitor and understand the relationship between fatigue accumulation and recovery. For example, I can watch fatigue accumulate from 5% to 7% to 10% and drop back down to 7% from an extra recovery day. And I can reproduce this with near perfect precision. See the 3 x 3 x 3 test discussed in the PB Science video I posted above. It allows me to play with intensity, duration and frequency at multiple levels to tune my training according to my ability to handle load at both the microcycle and block level. For example, I might find that I can handle more overall load with a higher ramp rate but more frequent rest weeks...or vice versa.

You can't go wrong using the 5-10% but this is also a range for the purposes of guidance based on a lot of internal data so your magic number might be 4 or 11 or 8. Or maybe there is no single magic number. The point is to open your eyes and begin the process of learning about how your body individually is responding to training and the more data you accumulate the more you will know yourself.

Another thing to consider is that deliberately allowing fatigue to accumulate before dissipating it is a valid approach to training.

My prescription is designed to get people started but is not even close to fully leveraging the power of cardiac drift and longitudinal analysis.