Back to Work - Stored Breast Milk

I am unfortunately headed back to work with my last week out being next week. I have about 100 oz of frozen milk in my fridge. I feel like I should start using the oldest milk there and start rotating it out, but I am not quite sure how to do it. I usually keep 3 bottles in the fridge if needed if I head out of the house, as personally, I don't like to nurse in public. Sometimes I am able to go to a private area, which then we end up not using the bottle I brought, but other times we use the bottle and then I go to the car and pump. It is my understanding that going back to work and getting onto a pumping schedule for the day can lower my milk supply. Right now I get two extra bottles a day that I end up storing, but my freezer is getting a bit crowded. Just not quite sure what to do at this point. I don't want to stop getting the extra bottles, but am I over pumping? Should I try next week to imitate the schedule that I will be having at work? I want to be prepared for the babysitter to know how many ounces to give the baby and when, but of course with nursing, there is no way to know exactly how many ounces DD is getting each session. (Sorry for the long post)