While practicing, you want to be succeeding on 50-80% of attempts.
Fewer than that and you’re going to be confused and feel like it’s all luck.
More success than that and you’re not pushing yourself.
Via Daniel Coyle’s excellent book The Little Book of Talent: 52 Tips for Improving Your Skills:
Sensations: Ease, effortlessness. You’re working, but not reaching or struggling.
Percentage of Successful Attempts: 80 percent and above.
Sensations: Frustration, difficulty, alertness to errors. You’re fully engaged in an intense struggle— as if you’re stretching with all your might for a nearly unreachable goal, brushing it with your fingertips, then reaching again.
Percentage of Successful Attempts: 50– 80 percent.
Sensations: Confusion, desperation. You’re overmatched: scrambling, thrashing, and guessing. You guess right sometimes, but it’s mostly luck.
Percentage of Successful Attempts: Below 50 percent.