R1 and R3 should be about 1000 times greater than shunt resistance.
That would be R1 and R2.

Good clarification Kai.

Kais circuit goes for very high precision, my experience with 4-20 is that such does not exist and for that reason I would not go through the expense of .1% resistors, just 1% will do. Also I would insert a resistor (5%) at both ends of the 2 potentiometers to make the adjustment of zero and span easier.

Erik

Erik