  All stellar evolution models for nucleosynthesis require an initial isotopic
abundance set to use as a starting point. Generally, our knowledge of isotopic
abundances of stars is fairly incomplete except for the Solar System. We
present a first model for a complete average isotopic decomposition as a
function of metallicity. Our model is based on the underlying nuclear
astrophysics processes, and is fitted to observational data, rather than
traditional forward galactic chemical evolution modeling which integrates
stellar yields beginning from big bang nucleosynthesis. We first decompose the
isotopic solar abundance pattern into contributions from astrophysical sources.
Each contribution is then assumed to scale as a function of metallicity. The
resulting total isotopic abundances are summed into elemental abundances and
fitted to available halo and disk stellar data to constrain the model's free
parameter values. This procedure allows us to use available elemental
observational data to reconstruct and constrain both the much needed complete
isotopic evolution that is not accessible to current observations, and the
underlying astrophysical processes. Our model finds a best fit for Type Ia
contributing \simeq 0.7 to the solar Fe abundance, and and Type Ia onset
occurring at [Fe/H]\simeq -1.2.
