We're all trying to predict players ability to knock down shots in the NBA. It's not easy and it is likely none of us are mostly right. I try to look at form and the difficulty of the shots they take. I'm probably wrong in most of my assessments.
When it comes to this matter, there are two players I currently think about. Beal and Jaylen Brown.
Beal did not shoot well as a freshman. Scouts raved about his form and insisted he would be a great shooter in the NBA. They were right.
Brown could get to the rim as a freshman, but couldn't finish. He couldn't finish in the summer league either, but he was actually pretty good at finishing as a rookie.
These things are hard to predict. In my unprofessional opinion, I think Tatum will be able to knock down a lot of the difficult shots he takes, not as a rookie, but probably in 2-3 seasons.