i made a weakaura recently for this, the string is
here
the goal was for it to pop up when my target went below 35%, regardless of whether i had a rune ready for it, and to show its cd after using it...
i'm not even mediocre-ly expert at making weakauras by any means, but i've had no problems with it so far ^.^
pictures, because why not
soul reaper up (target <35%)
soul reaper on cd