Going back to work tomorrow

I'm so anxious about going back to work tomorrow. I only pump an hour after he goes to bed (because he sleeps for 6 hours at night) and I've never been away him for longer than it takes to get a shower. Lol... I'm so nervous that I won't pump as much as he drinks while I'm away. I started trying to pump a little here and there since yesterday to have fresh milk for him tomorrow (instead of frozen) and it looks like I'll be leaving 16oz. Divided into 4-4oz bottles for an 8 hour shift... But I'll only get 2-15 minute breaks and 1-30 minute lunch break.... What are some tips to pumping at work to be sure to get enough? I tend to only get 4 oz. when I pump at night, but if he drinks 4 bottles and I only pump 3, I won't have pped enough.