The Rule of 100%

At the gym, regardless of your specific fitness goals, the number of repetitions you do is not nearly as important as whether those repetitions are performed to a moment of muscular fatigue. Not being capable of performing one more rep ensures that all of the muscle fibers responsible for moving that muscle have been engaged. If you feel capable of performing a few more reps at the end of a given set, you have not fatigued all of the type II fibers, which means you have wasted your time because you will not be training in the most efficient manner possible for your goal.

The same applies to any exercise, work, or project you're currently focused on. Only pushing it all the way through to the muscle (aka your ability) fatigue moves you out of your comfort zone and helps you grow. Playing the same song over and over won't yield nearly as many results for your skills as completing one at the peak of your abilities and nailing it.

Make sure to cross that 100% bar in your work to grow and succeed.