I fear, your question doesn't become much clearer with the block diagram. Reference current refers to the quantity z, output of the multiplier, I guess. But wha do you mean with "does one obtain the reference current"? Are you asking for the PFC operation principle or a particular measurement you want to perform? If the latter, which information you are looking for? In steady state, the output voltage will be equal to the setpoint and doesn't hold a particular information. The load curent would be more interesting and allow some simple calculation based on conservation of energy.
You may want to give an example of the said procedures found in "every book" to make us understand what you are actually looking for.
P.S.: Reviewing your last post, I understood that you are asking for the operation principle of the PFC control circuit. I hope, you understood it's purpose, forcing the current waveform drawn from the mains to a sine.
The output of the voltage error amplifier can be simply seen as a slowly variable quantity that tracks the power demand of the load automatically. The error amplifier has typically a PI characteristic. It's integrator action results in zero error in steady state.
A constant in front of the multiplier won't change anything, except for the control loop gain and still result in the same reference current. This is because the error amplifier will change it's output voltage until the necessary reference current is achieved. Loop gain is important to setup a stable controller, but the exact quantity can be ignored, when you try to understand the operation principle.
P.S.: I think, it's helpful to think about the conservation of energy aspect. For a particular output power, a specific input current magnitude is required. The control loop achieves this by varying the reference current.