Velocity(m/s) = RPS * Wheel circumference(m)
Wheel circumference(m) = Pi(3.14) * Diameter(mm) / 1000
RPS = RPM / 60
If targeted Velocity is 0.5 m/s, RPM is 60 (standard servo) results a Wheel circumference of 0.5 m, or 500 mm. This is way too big for a servo. Let's calculate the other way around. The wheels that I have are 66.88 mm in diameter, that means the circumference is 210 mm or 0.21 m. For the same targeted velocity, the RPM should be Velocity * 60 / Wheel circumference, that is 0.5 m/s * 60 / 0.21 m = 142.857.
Now to calculate the encoder pulses. For the 210 mm wheel circumference, to have a traveled distance of 5 mm per click, I need 42 clicks per rotation. That means 21 white and 21 black stripes, or 21 holes in the wheel. I would like to have more precision than that, so I will try to mount the encoders directly on the back shaft of the motor that spins 51 times faster (that's the gear ratio) than the output shaft.