The dephased part is counted but does not contribute to the visible phase signal. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Coherent and dephased shots split the budget
Out of 60 phase-sensitive shots, 45 remain coherent and 15 are dephased.
45+15=60
The dephased part stops contributing to visibility
Only the coherent fraction 3/4 contributes to the visible phase signal.