I haven't seen guides for arena increasing, but I think before increasing the seats you better rise the ticket prices near to the maximum, if still the arena becomes full then go for increasing seats. As the bleachers sold mostly then LT so make your increment balanced like, Bleachers around 70%, LT 18% CS 10% LB 2% some thing like that. Or simply you could add the seats that are always full.
hope this help you.