Lightning Surge deals 4 damage to any target.
Threshold — If seven or more cards are in your graveyard, instead Lightning Surge deals 6 damage to that permanent or player and the damage can't be prevented.
Flashback
(You may cast this card from your graveyard for its flashback cost. Then exile it.)
At the time this spell is resolving, it is on the stack and not in the graveyard. This means that you should not count this card when determining whether the Threshold effect occurs.