Being a dps is all about maintaining up time. So the BIG rule of that is to not sit on your GCD's to wait for something to reapply. So never hold a GCD to line up your dot timer, and don't just burn several oGCD's to heavily clip your GCD either. This will result in losing GCD casts during a fight which quickly adds up to lost damage.

And to simplify dots, you ideally will want to reapply shadow fang within two seconds of it falling off (either before or after) to ensure you are getting maximum up time on the dot tick.