Reducing the differential amp gain fits a higher current into the Arduino’s fixed 5 V ADC range:
Those are 1% resistors, chosen from the heap for being pretty close to what I needed. Given that it’s an LM324 op amp, we’re not talking instrumentation grade results here.
The same calibration run that produced the DAC plot gave these values:
The linear fit gives the actual current, as seen by the Tek probe, for a given ADC reading.
The trimpot controls the offset voltage at zero current; working backwards, ADC = 0 corresponds to 140 mV, a bit higher than the actual 90 mV. Close enough, at least for a linear fit to eyeballed data, sez I.
Working forward, the maximum ADC value of 1023 corresponds to 4 A, which should suffice.