Hi Rusty!
I used to express on an evening once I had had my dinner and a bit of a rest (???!!!??) and found that was when I could express the most. The amount you express is no where near the amount Harriot will get when she has a feed. babies are better than pumps at extracing your milk!

My DH used to give the expressed milk in a bottle for the evening feed though so I had had a break from BF to allow my milk to build up. Used to get about 3-4 oz off each breast (Avent hand pump).
Pumping in the bath (

) can work too if you put a warm flannel onyour breast it can help with the let down. Or if you are good at multi tasking you can pump the other breast while you feed Harriet on the other one.
As you say I was never sure how much Kaitlyn took from me but in the bottle DH used to give her last thing she used to take anything from 4 - 6 oz. Once she took 8 but that was unusual. I used to freeze batches of 4 oz to avoid waste and found that was a good amount. You've seen those bf milk pouches from Boots?
What kind of pump are you using? Some people find it better to hand express but I could never do that well. I used an electric pump in hospital which was great but bought an Avent hand pump and found it worked fine in the end. It is a knack though and quite tiring!
Good luck on going back to work and continuing feeding. I've just given up after a year of BF...sob!!!
There is a breastfeeding thread on here somewhere but you might have to scroll down to find it.

Laura Mc xx