Mixed feeding from six months: how much formula?

[MrsActon]

I wanted very much to EBF but after weeks and weeks of agonisingly slow weight gain (five weeks to get back to birth weight) decided to start supplementing as per pediatrician's advice. But because very keen to protect breastfeeding this was small amounts (30 ml or so) after each feed (8 a day, both sides), and I was also pumping four times a day, and at one point taking domperidone and various gruesome herbs/ teas. Tbh none of the attempts to ramp up production seemed to make much difference, and now at 5 1/2 months he is having about 400 ml of formula a day, 100 ml of expressed breast milk and lots of (presumably tiny) breast feeds! Not sure if amount of formula has gone up over time because he is bigger (about 8kg) or because breast milk supply dwindling?

Basically am confused by fact that formula manufacturers recommend increasing total ml as baby gets bigger, but breastfeeding sites like Kellymom say that total amount of breastmilk consumed roughly constant.

Sorry for long post. Going back to work in two weeks at six months. Would like to carry on breastfeeding as far as I can for closeness etc, and some immunity, but am going to have to give up on pumping and will be away from baby from 10-5. If I carry on doing 4-5 (presumably small because my supply rubbish!) feeds a day, how much formula should he have? And how many feeds a day? Because of wanting to carry on getting as much breastmilk in him as possible I have been doing lots of small feeds/ have failed to establish any schedule.