the higher the value of the upper resistor, the higher the output voltage, considering that you keep the lower one the same.
the diagram doesn't show the negative feedback loop that probably exists inside the controller.
Remember the law...opamp with negative feedback drives its output until its inputs are the same voltage.....there you have it.....if the ref is 0.6v, then make your voltage divider give 0.6v to the error amp input when your vout is your required voltage.