ABOUT HOW TO MAKE A DC VOLTAGE REGULATOR 0-50V AT HOME

Could you you should demonstrate how in the above mentioned circuit the value of R4 is calculated using Ohms law; i.e. how is the determine of 0.6 arrived at, and by using this calculation what is going to the shorter circuit current be at any provided voltage placing? The DC from and SMPS is not any distinct from a rectified DC from the trafo, in