Conflagrate

If X is 0, Conflagrate has no targets and deals no damage.

If a spell has {X} in its mana cost, include the value chosen for that X when determining the mana value of that spell, even if it was cast with flashback and no mana was spent on X.

"Flashback [cost]" means "You may cast this card from your graveyard by paying [cost] rather than paying its mana cost" and "If the flashback cost was paid, exile this card instead of putting it anywhere else any time it would leave the stack."

A spell cast using flashback will always be exiled afterward, whether it resolves, is countered, or leaves the stack in some other way.

You divide the damage as you cast Conflagrate, not as it resolves. Each target must be assigned at least 1 damage. You can't choose more than X targets and assign 0 damage to a target.

You must still follow any timing restrictions and permissions, including those based on the card's type. For instance, you can cast a sorcery using flashback only when you could normally cast a sorcery.