There is not a specific setting for your mouse for you to become amazing at aiming. That said, there is a reason many of the pro players stay in a specific range and that range varies between each hero as well. This website is a good starting point at choosing a mouse setting that suits you.
Mouse sensitivity/DPI settings in the end is a personal preference but in general, lower relative sensitivity equates to more precise and controlled mouse movement. There are plenty of resources available on this subject and I advise you to do some research to find a suitable setting or starting point to adjust from. If you are still unsure, I can safely recommend an in game sensitivity of 4 at a DPI of 800. This a good average setting for you to adjust from.
Wrist Vs. Arm Aiming
This argument also ties into your mouse sensitivity. Wrist aiming reduces your range of motion and benefits from a highers sensitivity while arm aiming increases your range of motion and benefits from a lower sensitivity. Basically the difference between the two is the pivot point of your movement. Wrist aiming’s pivot point is simply the bottom of your wrist sitting on your mouse pad and does not move much. Arm aiming’s pivot point is the bottom of your forearm or elbow. There are plenty of YouTube videos illustrating the difference between the two and the hybrids in between. I advise you to do some research and test which is more advantageous to you.
Fairly self explanatory but often overlooked. Proper posture such as sitting up straight, shoulders back, and having the center of the monitor at eye level will help you improve more you imagine. It will be uncomfortable at first developing proper poster but the end result will help prevent health problems especially during long gaming sessions.
I am referring to physical health and physical exercise here. Physical exercise can increases your mental acuity and focus. Take breaks during long gaming sessions. Go for a walk. Exercise your arms with a lightweight dumbbell and work on finger strength with a stress ball. Physical exercise will reduce fatigue and stress, increase your control over the mouse, and elevates your mental health for better focus.