I'm looking at R4 in the circuit and it looks like the zener diode D8 at just over 1ma. Is that the feedback ratio of U1? RV1 seems to determine the output voltage. Adusting the offset to produce the desired voltage isn't familiar. Can anyone help explain this?
The BZX79C5V6 zener diode that I recommend for D8 is 5.6V when its current is near 5mA.
R4 also has 5.6V across it so its current and the zener's current are 5.6mA.
RV1 is the trimpot to adjust the output offset voltage. It varies the output voltage plus and minus a small amount. When the voltage setting pot is set to zero then RV1 is adjusted so that the output is exactly 0V.