Most players are guessing when it comes to free throws!
I feel the problem is that most players are guessing when they shoot, be it jump shots or free throws. The key is to get your motion to be "repeatable," something you can do over and over, almost in your sleep. With practice, your motion will become accurate and consistent and all you'll have to do is "do your thing" and the ball will travel the same, practiced distance every time. Accuracy comes with the Release being an arm push with a relaxed wrist and hand, directly in line with the target. If the wrist and hand don't interfere, the ball goes exactly in the direction the arm creates. Consistency comes from the bigger muscles of the lower and middle body driving and stabilizing everything, high and soft. And Repeatability comes from a Release being a simple, end-of-the-arm, pushing action that you can do the same every time over and over, without thinking or judging. A "no brainer."
In this way the free throw can become an easy shot! It can become a "sure thing," rather than a "guess!" Since everything can be constant, you can learn to do the same thing over and over. Constant distance, constant leg drive (with practice,) constant Release action, and constant approximate height will give you a ball flight that is predictable. Since there may be fatigue to weaken you or adrenalin to strengthen you, height can be the final adjustment, but otherwise it can be a simple, completely repetitive action.