Usually when you don't have find weakness up you have no reason to cast much other than if your going to cap energy cast backstab, apply hemo (don't refresh it when Find weakness is up and dont refresh it until its run out) keep your rupture and silce and dice up.
When Shadow dance gets to like a 15sec CD, stack your CPs up to ten with anticipation then rupture slice and dice and go ham with shadow dance and ambush. You need to remember that in a raid combo points will generate themselves through honour among thieves, so as long as your not capping energy you don't need to really cast much.
Try to maximise your up time of find weakness it's where most of your damage comes from make sure you utilize your vanishes as its about to run out and end shadow dance with an Ambush. Like already said you wanna fit as much in to shadow dance as you can which is why you pool energy for it and combo points to make sure you don't have to refresh slice and dice or rupture while shadow dance is active.
Sorry for the wall of text :P