There is a post in the above forum by someone who works in the industry. They say use a 0.1uF in parallel with a 10uF capacitor. If there is still jitter, place more. There's really no calculation to do, unless you want to also calculate according to the exact distance (in millimeters) your capacitor is away.
As to the comment of diodes in this discussion... you are correct that you usually want flyback diodes! But most hobbyist servos are actually a DC motor with a speed control mechanism. That probably already has a flyback diode, and the use of a decoupler is to protect the bounce going towards the speed control mechanism. Please see below youtube for a dissection of a hobbyist servo.
