Hi,
I want to use the TNY274GN along with the PC817A photocoupler to control a flyback circuit.
The attached file, taken from following address, shows this configuration for 5 V output.
https://ac-dc.power.com/sites/default/files/PDFFiles/rdr295.pdf

My question is how is the resistor R9 selected?

Please note:
In the datasheet of TNY274GN is written that the transistor of optocoupler pulls current out of the EN/UV pin of TinySwitch. The MOSFET switching is terminated when a current greater than a threshold current is drawn from this pin. Switching resumes when the current being pulled from the pin drops to less than a threshold current. The threshold current is between 90 μA and 115 μA.

However, if we consider the following equitation based on the attached file:
Vo=R9*I+Vf+Vka, as V0=5V, Vf≈1V & Vka=2.5V
If=I-I1 ===> If=(Vo-Vka-Vf)/R9-Vf/R8===> If=1.5/47-1/1000≈31ma

Then, the current drawn from EN/UN pin is 31ma*CTR, where CTR is Current Transfer Ratio of the PC817A that is between 80-160%.
The calculation shows that the current drawn from EN/UN pin is much higher than the threshold current!!