How to not waste formula

Any advice on how to waste less formula? My daughter is not super consistent in how much she eats at each feeding, so we're always guessing whether she wants 6 oz or just 3 oz. This means we end up wasting a lot of formula since often we make more than it turns out she wants, and the pediatrician told us you can't save what's leftover after starting a feeding. It's getting to be really frustrating! We just started formula feeding our daughter about a month ago and we're very aware of the cost, so we hate to waste any.