Negative feedback starts by subtracting a measured output from a target. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
The output measurement is a separate value
The target is 8 volts and the measured feedback is 5 volts.
T=8V,M=5V
Negative feedback subtracts the measurement
Subtracting the measured value leaves an error of 3 volts.