Reorx_Holybeard wrote: »It's ~17.75% of the offhand weapon's damage. So a gold weapon with 1335 damage will add 237 weapon/spell damage in the offhand. This is 15.1% of the total damage of two gold weapons (1335+237=1572 * 15.1% = 237) so perhaps that is where the confusion originates.
Note, just tested this on PTS to confirm in case it changed recently.