I try not to worry about that too much. if the monsters are dying, I don’t really care where the damage is coming from. but for those builds that rely on 1 or 2 Skills to do all the damage, then that knowledge is important. if you have the Crystals for an experiment, put a WD%, WD+, ED%, & ED+ on one of your weapons, one at a time, write down the damage your Skill does, (Comet or Meteor if you are using a Staff, for example) on the Stat Page, with no other affixes on any other items. then see how the damage changes when you do combinations of those 4 affixes. just use Epic affixes, as those are the easiest and cheapest to get. if I remember the damage calculations, WD% is calculated first, then WD+, ED%, then ED+ last. if your Power Stat is low, your Damage+ affixes could give you better damage than the Damage% affixes, when you put more Points into Power, eventually the Damage% affixes start doing more damage than the Damage+ affixes.
example: if your Power is giving you 1,000 damage, a +100% WD would give another 1,000 damage for 2,000 total. if you had that 1,000 damage with the +5,000, that is 6,000 damage total. with both, 7,000 damage. if your Power was giving you 10,000 damage, with +100% WD, you are doing 20,000 damage, with +5,000 WD, only 15,000 damage, with both, 25,000 damage. when you add the ED damage, the +20% would use the 25,000 damage, so now 30,000. +5,000 ED would be 25,000 also, so at this point they are the same. if that damage was higher, then the 20% would be better. if lower, the +5,000 would be better. if you didn’t have WD % or +, the ED % and + would only work on the damage you get from the Power Stat, so with the 1,000 + 20% ED = 1,200 damage. 1,000 + 5,000 ED = 6,000 damage. WD % and + only works on the weapon it is placed on, but ED % and + works with both weapons, no matter which item or pet they are on.