On Blitz, like Thresh, Position is KEY.
If they are under Turret with Wave... why try to pull?
They already are at a disadvantage on ability to CS... let them suffer as your ADC pokes them for trying to Last Hit and gets Free Damage on the Turret.
So knowing WHEN is as important as knowing WHERE.
There are 2 Times where Blitz will literally not miss... that is Chasing and Being Chased. As most will run in a straight line, as it is the shortest point. If they 'weave' for a POSSIBLE Blitz Pull, then your ADC will catch up by running straight ahead to catch up. So you just fire it right into their Back and hope they don't have the Reflexes to Flash forward and dodge (and if they do, a Hook for a Flash is a 'Win'). If they are chasing you, use that No Turn Radius to simply fire the hook into their face, Ult and Knock Up while they get melted by your ADC.
Another great time to hook is when the Support leaves the Wave to Ward... catching them coming back into lane is super easy then as they have to come around the Wall, exposed as soon as they exit the Brush.
Yet another time is Under their Turret, as your ADC is Last Hitting the final Minion of the Wave... try to time it to land directly after the Minion dies, so the 'shield' they have is gone. You can catch them off-guard at that moment, right before they move away.
In Team Fights, be careful of WHO you Pull. You don't want an Amumu or Zyra or Karthus in the middle of your group, especially if they can Zhonya's before you guys delete them... because they all have AoE Abilities that will jack up a team, allowing their Team to follow up. Also a Zed can be bad news, as if you don't delete him he's now CLOSER to your Carry to dive then Ult.
That being said, when Enemies are Grouped and none of these 'Do Not Pull' Champs are in the mix, just aim into the mass. Sure, some will dodge, but it's Fish in a Barrel and you'll grab some one for sure.
And advanced method is the 'Lead'. Say Amumu is flying into Baron Pit. Well, they can Hop the Wall out, or they can try to Dive Out the Front. So don't toss the Hook into the AoE Stun... it serves no purpose. Look for the Flash, and pull that guy when he lands, as he'll likely but up against terrain and have limited options AFTER the Flash. Same with Snares. Morgana tosses a Snare, they have to go Left or Right (as hopping it takes extreme timing and diving away from it just means it can still land if not out of range). Let them Land and fire it at them. It's super hard to juke immediately after a Dodge.
If at Higher Elo, then sit down with YouTube and watch a crap ton of Mad Life landing Thresh Hooks... to see how he anticipates WHERE to throw them, even in the blind. Certain Champions that have a 'reboot point', like Ekko, LeBlance and Zed (double, mark and shadow) are super easy. They WANT to dive in, then jump out (especially Under Turret). Don't aim where they are, but aim where they WILL be and pull them right back under. Takes a lot more skill, but worth it if you can master it.
Hope these ideas help... but the main point is, you can not 'always' be in position to Hook. So do what you can to maximize your chances when you are.
