Determine your preferred FPS sensitivity and measure it in cm/360 (how much horizontal distance do you need to move your mouse to complete a 360 degree turn).
Try moving your mouse at different speeds as close to the same distance as possible each time. Does your mouse land on the same area each time? If so, your sensitivity is configured and you should have a consistent experience, allowing you to properly develop the muscle memory needed to aim in an FPS game. Do you turn in excess of 360 degrees? Mouse acceleration. Do you turn less than 360 degrees? Negative acceleration.