If you're interested in timings, Death and Taxes do some very good written guides. The moveset is listed out by time in seconds, which you can then use to map out your cooldown activation. If it's a newer fight, the easiest way is to either watch a clear or your own replays and measure out the timings from the video.

The presence of a cast bar doesn't really matter. If you're mitigating a single big hit, try to activate the cooldown as early as possible to ensure that your cooldowns are back up faster (i.e. try to activate Shadowskin early so that it catches the tankbuster towards the end of the 20s, rather than at the start). This is especially important if you're playing DRK on a physical fight, because you don't have the luxury of Sheltron.