As far as lay out - This won't help you as you are cross barred. I won't go into depth on how you should be playing your class as it depends on the situation (Dungeon / Raiding) but you want to lay out so your cool downs are accessible and that Dart Arts is easily reachable as you will be using it a lot.

I group my combos together, my aoe's, my mana buffs, and my cool downs.

As far as flow - Dark knights are in a weird spot. It's very button pushy and speedy. But a lot of fun. (Design i'm sure)

Basic guide
https://docs.google.com/document/d/1...N7pMgnefU/edit

Screen of my layout
https://imgur.com/a/pL0D6

Good luck and keep practicing! It'll hit you with an ah moment.