My dh and I worked out how much we needed for a 24hr period (give or take) and we'd make up a 1L container and just put it in the fridge. If we needed more, we also always had a bottle of cool boiled water in fridge and we'd heat that and add formula. We found this a life saver at nights. You didn't have to think and measure, you just pour and heat gently. If we went out, we'd take along a small esky with the sterile bottles in it, and the bottle of formula. We'd also take along some extra cooled boiled water JIC and a small container of formula (or a small sachet) JIC. It worked well for us, but I can see how what u guys did worked well for you! I started out like that, but Jess and Kat were both impatient feeders and they'd scream non stop till u fed them so we used the quickest mode we could.....
We made sure we steralised everything and boiled everything as u must. I think it's 6 of one and 1/2 dozen of the other IYKWIM? Whatever works best for you, do!