Play spell damage before counting the final route
If a minion or aura gives spell damage, count your hand as if that source is already active. Then check whether the source itself costs mana or board space.
Sequencing guide
Spell damage lethal order in Hearthstone
Spell damage lethal is one of the easiest routes to miscount because the same card can be worth different damage before and after the spell damage source is played.
Count each spell separately
A +1 spell damage bonus applies to each damage spell. Three small spells may gain more total damage than one large spell.
Watch non-damage spells
Draw, discount, and discover cards may be correct first if they find extra damage. But if the route is already lethal, extra actions can create mistakes.
Confirm the target order
Some spells need to clear taunt or Divine Shield before face damage is possible. Count the spell damage bonus only on spells that actually deal damage.
Common miss
Players often count the boosted spell damage, then forget the mana needed to play the spell damage minion. Always count damage and mana in the same pass.