Masking a cell removes that cell from the active coverage ledger, even though the physical grid still spans the original length. The subtraction is exact for the stated active-cell count and pitch; real masks can include dead or noisy channels, partial edge response, changing thresholds, and readout ambiguity.
Active coverage comes from active cells, not total cells. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Masked cells reduce active length
Only 3 of the 4 cells are active, so active length is 6 meters.