you will notice in the character screen that there are 3 tabs. the 2nd tab shows the dmg breakdown of your left/right click abilitys as well as weapon dmg.
flat dmg is only applied to weapon attacks, but not spells. %bonus damage applies to all sources except retaliation.
ah thank you, so much for the info! And I presume, other thing such as your gameplay also affect this. For example if you are hit and run player than attack speed may not matter as much for you?