A calibration line is trusted only inside its declared standards. 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 standards declare a signal range
The standard signals run from 2 through 14.
S∈[2,14]
Out-of-range inverse values are rejected
Signal 17 would invert to 5, but it lies outside the declared signal range.