Are yeu taking armour into account? Since that could be a large part of it.
Things like twisted fate and heroes with "on next hit" abilities make it very hard to gauge the power of sheen/triforce due to the secondary effects also being applied (usually more damage).
If sheen is doing MORE damage than triforce though, on the same character, as in yeu upgrade the item and then do *LESS* damage to minions... then we have a problem.
I'd advise a practice game, with a hero that does NOT have an 'on next attack' spell, and test it against minions. This should allow the best comparison of sheen vs triforce.
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