Separate clear damage and face damage
Do not add every attack together at once. First assign enough damage to remove the taunt, then count the remaining attacks that can go face.
Board math
Taunt wall lethal math in Hearthstone
A taunt does not only remove face damage. It also changes which minions can attack, how much overkill is wasted, and whether spells should go face or into the blocker.
Track overkill
If a 7 attack minion hits a 2 health taunt, five damage is wasted unless the card has a special effect. Overkill is the silent killer of exact lethal math.
Check Divine Shield and reborn effects
Shielded blockers may require a ping before real damage matters. Reborn or summon effects can leave another blocker behind.
Choose spell targets last
Sometimes a spell should clear the taunt so minions can go face. Sometimes minions clear and the spell goes face. Compare both routes.
Fast taunt shortcut
Ask: what is the cheapest way to open the face? Then count only the damage left after that route.