I'm not a professional Pantheon player but here is what I've found:
First off you got it right by coordinating with your teammates CC, and getting rid of all of their escape is typically good (unlessy you're talking about abilities in which case they just have to wait a few seconds and do it again)
It usually works better when the teams are engaged too because then you give your opponent the tough choice of either finishing the engage but taking damage from your ult and facing a pantheon, or running away and taking free damage as they run. Another one is locking them in, which requires teammates that can take them on if they would rather avoid your ult than run through it. Example would be a low health mid laner and your mid lane is chasing, jumping in front of the path that the opponent is running will cut them off leaving them to either run back and die or die to you. I'm asuming that when you used it to chase someone who is escaping and failed it was primarily because you were in a 1v1 scenario, but even if your damage doesn't go off forcing your opponent to make tough decisions like those are what really makes the difference. If the target has a way to get out without any penalty don't use it, simple as that. Also typically you want to be away from a team fight when you use ult so that it doesn't get canceled. Even if the enemy team didn't have any way of canceling your ult, you may be better off saving it and doing even more damage with your other abilities and auto attacking: remember pantheons ult takes time, and time is damage. Id say another trick understanding his ult is typically you're not trying to get kills with it alone. Yes there are times where there are close team fights and a pantheon ult will kill one or two of them, but those are the obvious ones, most of the time you'll be coordinating it with teammates to take down a few escaping opponents or protect your dying teammates from a losing team fight byt intimidating them. Don't dive teams, and don't dive turrets (often), try to focus on "does this help your team get more damage off to win a fight and/or get an objective" rather than "will I get a kill or will I die". Hope this was the answer you were looking for!
P.S. another step ahead of this would be to ask is panths ult even really useful? As I have explained most of the time you give your opponent the choice in most senarios. Granted this sometimes leaves them with a lose lose choice but if they have the choice then they have the opportunity to gain an advantage if they make the right choice. It's all about thinking ahead and making sure that doesn't happen and not wasting your ult unless you know for certain it will work for the benefit of the team.
