For your performance to be perceived as great, you have to be performing the top 30% of your abilities, but solidly within that ability. That's really all you have to do. Music, as a performing art, is perceived subjectively, not objectively. The audience doesn't pull out their metronomes and measure how fast you're playing or whip out their tuner to see exactly what note is being sung. They groove to the flow, baby, classical, jazz, it doesn't matter. So a performer who is going for it and nailing everything they do will be perceived as better than another performer singing higher, playing faster, who misses some of what they're attempting. That's why when preparing for a single performance you should cut off improving the piece at a point where you have plenty of time to consolidate your gains.
Progression of issues for the vocalist: first ear, then vocal, next musical, last artistry. As you master one you work on the next.