At a late time, the magnitudes of the objects are

           SN    V = 16.4

            P    V = 21.1

   To compute the ratio of their apparent brightness,
we calculate

        intensity of SN         -0.4 * (16.4 - 21.1)
       -----------------  =   10
        intensity of P


                          =   76


    So at this time, the SN is about 76 times brighter 
than star P in the V-band.   That means that if one collects
the light from both stars, but attributes all the light
to the SN, one will end up with an error of about 1/76,
or about 1.3 percent = 0.013 mag.