How do people calculate per diem interest on consumer loans?

all of our loans use a 360/360 schedule.

In the past we have used the following calculation to determine per diem:

loan amount x interest rate / 360 = x # of days

I have also seen:

loan amount x interest rate / 12 / number of days

Each of these gives different per diem amounts.

If we are using a 360/360, should we be using 365 for calculating the per diem interest amount?

we use 360/360 which assumes every month is a 30 day month

balance * rate / 12 / 30 = per diem

we only use 365/365 on our daily interest loans