A blocked arm is accounted for as lost counts, then the surviving arm is split. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Blocking one arm removes those counts
The first split still makes 4 upper and 4 lower, but the blocked lower arm carries 4 lost counts.
Nlost=4
The surviving arm splits at the final splitter
The surviving 4 counts split into 2 bright and 2 dark. Interference is not claimed when one arm is blocked.