You should NEVER sell out your arena. 90ish % is optimal.
Think about it this way: assume that lowering the price will sell out your arena, but it will also decrease the return on every seat that you are normally selling.
So, if you normally sell 3700 out of 4000 bleacher seats at $10, and you drop the price $1. You lose $3700 ($1 x 3700 seats) and gain $2700 ($9 x 300 new seats) for a net loss of $1000.