Okay, I've tested this in PvP, with jeweled helms, just to be sure that it isn't a display glitch. The damage dealt is more or less what the Character Screen says it should be, which means that Ragnarod is correct.
* Jewels with only +ED% and no +Min or +Max damage will work as expected. That is, the +ED% will add to the item/skill multiplier, in the same way a Fortitude's +ED% would.
* Jewels with +ED% and +Min damage will only apply their +ED% to the total Maximum damage.
* Jewels with +ED% and +Max damage will only apply their +ED% to the total Minimum damage.
So, a 40%ed / +15 Max damage jewel in a helm will add +15 to your maximum damage, and apply an additional +40%ed to your minimum damage. Whereas a plain 40%ed jewel will apply the +40%ed to both minimum and maximum damage.