A transverse signal is visible only through the coherent bucket. 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 transverse budget splits into two buckets
The initial transverse budget is 12. Coherent signal keeps 9 and the dephased bucket holds 3.
9+3=12
Only the coherent bucket is visible
The visible signal index is 9, so the toy coherent fraction is 3/4.