Hi NC,
Sorry for the late response. I have not received the notice for your question.
since the turn ratio of the main transformer is 2:1. when the Vin is 48V, the voltage plateau of the secondary side sw node is 24V, so the R1 in the daughter card should be 23k to get a voltage of 1V to ACSNS pin. when you modify the Vin to 48V, the voltage plateau of the secondary side sw node is 12V, so the R1 in the daughter card should be 11k to get a voltage of 1V to ACSNS pin.
however, since the switching node is noisy. there are huge ringings especially at heavy load. if you add an RC filter to filter the ringing, the Vin sense accuracy will be much better. please refer to the circuit below.
