honestly, it's easily confirmed with someone who uses offline hero editor, which I dont. Simply make 2 jewels, both with the same +damage but different % enhanced damange. put it in a helm. swap between them for echo. if the dmg is the same, despite the jewel having different ED%, then it is bugged. if the dmg is different with the higher % ED jewel being higher, it works.
i feel like no one has tested for over a decade, and i don't trust hero editor?
you would need a target dummy and something that actually shows exact damage numbers and that doesnt exist. people say they test maybe you could online with another player and punch but min-max damage would mess up results so alot of testing would be needed
be nice.