I would stick to the original circuit but add two zener diodes back to back in series with the incoming AC. Using a resistor alone will not give consistent results, the detection voltage will depend on temperature and component tolerances.
Use the property of Zeners that they conduct like normal diodes when forward biased to cause a predictable voltage drop. For example, if your supply was 240V and you use two 100V Zeners, the opto-coupler would work normally down to 140V before starting to switch off.
Brian.