If I was playing and I had to use the Audience, I'd be satisfied with a simple majority of the audience went with the answer I had, ONLY if the next closest polled option wasn't within 15 percentage points of the most polled option.
I wouldn't play right away if it was a tight plurality, if the Top 2 polled options were very close to one another i.e. within 5%, or if I was at the last 3 questions of the game.
Example: If the top polled option was around 40% but the next polled was between 35%-40%, then I'd doubt the audience as then, it becomes almost a coin flip as to which answer was the most sure of the two. If it is too close to call or god forbid the Top 2 are TIED, then I'd have to walk away if it was the only lifeline I had left because I wouldn't be totally sure that the audience knew the answer or if I had other lifeline(s), I would have to use them as I wouldn't be too confident to risk it IF I had just played the Audience and if it was that tight.
Steve Perry is the poster child when it comes to my example. The top 2 answers were nearly an even split with neither getting 40% of the audience vote, and he had to burn his other 2 lifelines at the final hurdle, with the 50:50 in particular actually eliminating the Top 2 polled option. Even after all that by phoning his friend, Perry walked away with half a million and the least polled option ended up being the right answer.