Hiya
I wouldn't expect them to take their usual amount of formula for each cluster feed (i.e. 6 oz bottles if that's what they have 3/4 hourly). The idea is to get more in over all. For example on a 4 hour schedule:
without cluster feeds:
7am - 6oz
11am - 6oz
3pm - 6oz
7pm - 6oz
11pm (DF) - 6oz
= 30 oz
with cluster feeds:
7am - 6oz
11am - 6oz
3pm - 6oz
7pm - 6oz
9pm - 5oz
11pm (DF) - 5oz
= 34 oz
So with the extra feed you're getting a total of 4 extra oz into them.
Hope that makes sense (figures were just arbitrary btw)
love