As a general concept, you want to configure the internal phototransistor to respond to the internal led, so that it delivers the correct volt level to the feedback pin.
You must create the proper amount of current going through the led. If output voltage changes, then it must cause led brightness to change.
In regard to the phototransistor, you install a resistance on its collector leg, which creates an output voltage that is usable by your control circuit. This must go up or down with led brightness.