Supplementing question

Marianne
My milk is running low, which is making me really sad. But I don't have much of a say in it. LO is 4.5 months and doc said to consider supplementing with formula. I have about 50 oz frozen I can use too. So here's my question:
Do I use up all the frozen milk first and then switch to formula after I'm out? Or do I alternate the frozen stash with the formula so the milk lasts longer and she continues to get breast milk beyond 5 months?