I want to use a buck converter on my custom PCB for powering the board which steps down 12 V DC to 5V constant DV output. 
I came across a Buck Convertor in market Mini360 Buck convertor which can do the stuff but there is potentiometer on it which can be adjusted to vary the output voltage. 
Below you can see the schematic for the buck convertor and here I plan to remove the potentiometer and put fixed resistors to get 5V constant output.
[![enter image description here][1]][1] 
[![enter image description here][2]][2]


Can someone help me in how to put fixed resistors in place of Potentiometer and how to calculate the value of resistor ?



  [1]: https://i.sstatic.net/UiWIx.png
  [2]: https://i.sstatic.net/IrdDJ.png