optoelectronics - How is optimum series resistor of an optocoupler sized?

I want to learn how to size the series resistor for this optocoupler which has a high CTR.

I will use Arduino's 5V digital output pins for eight of this coupler so I really want to source necessary enough current from each pin.

What is the method to calculate the optimum like series resistor(R1 above) in this case?