Optocoupler Turn On: Varying Emitter Voltage

[likephysics]

When exactly does a opto coupler turn on, if the emitter voltage is varied.
See attached.
Input voltage for the opto is 5V. Drop across the opto is 1v. Total current flowing is 4mA
The Vcc on transistor side is 12v.
What should the emitter voltage be, so that the transistor turns on.

 

[NascentOxygen]

I'd say it can turn on when V_CE>0.2V