A fixed net count shows how calibration gain changes reported output. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Gain one reports the net count
Measured 12 minus dark 4 gives net 8. Gain 1 reports output 8.
1⋅8=8
Gain two doubles the calibrated output
Gain 2 maps the same net count 8 to output 16.
2⋅8=16
Gain three triples the calibrated output
Gain 3 maps net count 8 to output 24. No rail or nonlinear response is claimed in this scan.