I am trying to modify a micro servo for continuous rotation, but I am not sure how to center the servo's potentiometer without having to use a servo tester. The micro servo that I am using is "SG90 Micro Servo Motor 9G".
A hobby r/c servo is centered when the pulse width is (nominally) 1.5mS, or in the case of a continuous rotation servo, when the servo is not moving. i.e. the servo motor is perfectly idle with no current draw.
Simply connect the servo to a receiver/MCU with an analog input and adjust until the servo stops moving. No need for a servo tester.