breast milk and transitioning to formula?

m&m2006

i'm weaning myself off the pump and moving onto formula.  i have a little bit of a freezer stash.  is it better to give LO solely BM until it runs out, and then switch to formula, or do a mix so that she has a little BM every day for a longer period of time?

she already gets a little formula mixed with BM in her bedtime bottle.

thanks ladies!