The amount to pay for {X} is chosen as the triggered ability resolves. No player may take actions between the time you pay {X} and the time Hero of Leina Tower has X +1/+1 counters on it.

Heroic abilities will resolve before the spell that caused them to trigger.

Heroic abilities will trigger only once per spell, even if that spell targets the creature with the heroic ability multiple times.

Heroic abilities won’t trigger when a copy of a spell is created on the stack or when a spell’s targets are changed to include a creature with a heroic ability.