Do not belive everything on the net.
CTR are important to calculate with. Look at the 4N27, 10% up to what I have seen as max, 600%.
The solution with the zeners are not optimal when the input signal varies 20 volts (60-80)
I have read earlier in this tread why the design was made like this.
Was it nessecery to let voltage in the span 0-50 volts be absolutly off.
With only a resistor, 0-10 volts can be off and over 60 on.
Pelle