Problems sizing series input resistor for this optocoupler
The 4N26 is a really cheap crummy optoisolator and has a guaranteed CTR of only 20% at 25&deg;C. It will be worse at temperature extremes and with LED aging. So you would need a minimum of 80mA, but more like 120mA to be safe on CTR,  but that's too much for the optocoupler LED. Note that the curve you show is normalized so CTR drops to more like 12% at 20mA. 

4N36 has a more reasonable 100% CTR guaranteed so maybe  you can choose 25mA drive but that's still a bit high for the Arduino and for good opto life. Or put a transistor on the SSR side to drive the SSR. 

Note that the SSR already includes proper isolation so you may not require anything more than a BJT or MOSFET to drive the SSR if you don't need to keep the 12V power supply isolated. In such a case, I would use something like a 2N4401.