I'm having the same problem! My solution is to pick a random activity and calculate how many minutes it takes to equal pumping 1 oz of breastmilk. I then multiply that amount of minutes by the number of ounces I have pumped. It's an imperfect method, but it's the best I've found.
How have you been calculating how much to lose? I've been estimating that pumping takes about 650 calories a day (from 20 cal/oz as well) and I can take 150 of those calories from fat stores rather than eating to make them up.
Congrats on your progress so far! My kid is 5 months old and I still have 20 lbs to go (30 lost already!)