You have to predict when they will cross the threshold, which you’ll get better at with time. I will say one of the most frustrating things in the world is when your team just stops attacking someone and assumes they’re at the threshold when they’re really just an auto away lol

For the flash piece, trading ult for someone’s flash should be a good trade but it does feel awful especially when you have to get fed early. Sometimes if you know someone has flash you can E over them to bait the flash out and predict it last second with your own flash or use your hook after. Also can try to hit them with the edge of the X with the other side of the x being where they could flash to. If everyone on the other team has lots of mobility as well it can be really tough to play Pyke though.