Moderate evidence

Rating a set by how many reps you had left is a usable way to gauge effort, and gets more accurate as you get more experienced.

In practice

This is what the effort rating after each exercise is for. It's how the app knows the difference between 'made the reps comfortably' and 'barely survived' — two sessions that look identical in the numbers but mean opposite things for next week.

What it doesn't say

Accuracy is best close to failure and with heavier loads. On light, high-rep sets, and among untrained lifters, estimates drift a long way from reality.

Source

• Zourdos MC, Klemp A, Dolan C, Quiles JM, Schau KA, Jo E, Helms E, Esgro B, Duncan S, Garcia Merino S, Blanco R (2016). Novel resistance training-specific rating of perceived exertion scale measuring repetitions in reserve. Journal of Strength and Conditioning Research, 30(1):267–275. doi.org/10.1519/JSC.0000000000001049