Thanks @ldecenciere ! That formula works for all the combinations I've tried.
Spoiler: Final Plains of War damage is (swords - enemy shields) + (purple flower diff) or crits with the swords portion times two.
As @Garanor pointed out, it's very difficult to determine what the purple circle/flower symbol means in Plains of War. I've been trying to mix and match cards to try to determine what this number does. Maybe it's luck? Maybe it's crit? It's mostly been aggrevating.
Here's an attempt to figure it out:
Swordsman (4:1:2) does -3 or -6 damage to a goblin (4:1:2).
Swordsman with a well (4:1:6) does -7 or -10 damage to a goblin.
Swordsman with a Mana potion (4:1:7) does -8 or -11 damage to a goblin.
The formula I could be find that fits all 3 is an oddity (flower + 1 or flower + swords). I'd think the enemy shield factors in.
So throw in
Swordsman with a Chest (6:1:2) does -5 or -10 damage.
I can't find a formula that matches these 4 cases.
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Yes, that should always make sure they are linked.