20-60ml after a feed is pretty good!
The average bf baby needs around 800ml in 24 hours, though the amount individual babies take does vary.
You've got quite a bit of time to bank some milk, so even if you only get 100ml or so a day, you'll still be ahead of the game once you're back at work and then you can continue to express during the day (and you'll get more if you're missing feeds). Plus you can still breastfeed when you're with your baby. Babies will often take more when mums there and just enough to get by from the bottle.
As she passes 6 months, then she can also start taking some solids and water to get by during the day, so you may find it gets easier and easier as she gets older.

Sounds like you're managing this really well and good on you planning ahead for your return to work

The ABA has resources specifically for mothers planning to return to work - there's a booklet you can purchase through Mothers' Direct, as well as the Breastfeeding Friendly Workplaces program, which overs support and resources to employers to help them help their employees (you!) on their return to work.