Dot clipping is no longer a thing thanks to the newly added pandemic affect for most classes! (If you refresh a dot at 30% or less duration it will add the dot instead of replace it) so try to refresh in that window when a higher priority spell isnt up.
http://summonstone.com/priest/shadow/
There is a great guide for spriesting that gets updated very quickly (missing the boss guides cause hes in mythic progression atm but he will have em up soon)
You are only at 20% crit so you should have a ton of dead space in your single target rotation to refresh them but generally only let them fall if if MB/death is up (realistically this should never happen with the pandemic affect because you can refresh them early)
Cascade on any fight where there are spread out adds (majority of the fights in hellfire) halo for single target/adds are all clumped (fel lord/tyrant/iskar)