How can I estimate the initial luminosity of a SNIa given initial mass of Ni-56?

Nickel 56 decays to Cobalt 56 via electron capture decay, with a half-life of 6.1 days and a decay constant of $lambda = 1.31times 10^{-6}$ s$^{-1}$.

About 1.75 MeV of energy is lost as gamma rays and a further 0.41 MeV in the form of an electron neutrino (Nadyozhin 1994)

Let's assume that we are talking about the period of time after the initial detonation, where the fusion energy of carbon and oxygen is released and is sufficient to unbind the white dwarf.

Let's assume that apart from the very first few seconds of the supernova, that the envelope is transparent to neutrinos, so that energy is lost.

Let's further assume that the gamma rays are able to thermalise rapidy in the envelope, that the energy is able to diffuse to the "photosphere" of the expanding fireball in $< 6 $ days and that the work done in expanding any ejecta is also negligible. This latter may not be justified in a type Ia supernova.

The decay equation $N= N_0 exp(-lambda t)$, means the rate of gamma ray energy deposition will be $$ frac{dE}{dt} = 1.75 lambda N_0 exp(-lambda t) {rm MeV/s}, $$ where $N_0$ is the number of nickel nuclei you begin with.

Let's assume about $0.5M_{odot}$ of Nickel is produced (see Childlress et al. 2015, incidentally, this paper illustrates how complex this question really is). This means $N_{0} simeq 1.1times 10^{55}$.

Thus I make $dE/dt$ (assumed to result in the emergent luminosity), when $t=0$, to be $2.5times10^{49}$ MeV/s, or $10^{10} L_{odot}$.

This value is about a factor of two higher than the average type Ia supernova peak bolometric luminosity measured by Scalzo et al. (2014), which were well modelled with nickel masses of around $0.5 M_{odot}$. So I would suggest that one or more of the assumptions above (probably the one about no work being done on the ejecta) is a bit flaky. However, I also notice that the peak luminosity occurs about 2 weeks after the initial rise, so I would suggest that the "instant" reprocessing assumption is also incorrect and that the released energy is smoothed out somewhat so that a lot of the nickel has already decayed by the time you get to peak luminosity.