Just going to add:
Don't focus so much on your double haul as you do on good, hard stops and tight loops both backward and forward. Shooting line and adding a double haul (to me) are secondary to this IMO. The double haul is excellent for avoiding fatigue when you're casting all day and it certainly adds line speed and facilitates shooting for distance, but you don't want it to be a crutch to compensate for wimpy stops or wide loops
That's my take -- just an opinion.