Current sense op-amp with integrated shunt vs expensive resistor
If you want o build a calibration tool, you tool should be more accurate than the product you are attempting to calibrate.
You seem to be making a meal out of sensing with relatively high value series sensing resistors. You should be trying to minimize the sense R.

You could start here or here for pointers on how to measure accurately (ignore the fact that the links are about measuring low currents). These will give you a much better understanding of the accuracy and performance you might achieve. 

One great article to ponder is this, on the uCurrent Gold. 

The basic design could scale to many amps with ease, and with high accuracy and very low burden power dissipation.