A good way to eliminate waste with seeds, if you are home and can keep an eye on your bird(s) to make sure what their needs are, is to only offer 1 teaspoon at a time. Most birds will not consume more than 1-3 teaspoons a day, so why give them more? And if they are eating a variety of foods, there is no need to give them more than 1 teaspoons a day.
Regarding pellets, something to think about with pet Budgies is you should not feed them what they would eat in the wild because they do not fly (or move around) as much in captivity as they would free, so they do not burn as many calories or require as much protein. A Budgie in the wild will fly many miles a day to find food & water. At home, even if left out all day, realistically they may fly 1 mile. That said, you need to calculate your bird(s) activity level and fly time, calculate calories, and manage their weight to find the perfect food/pellet balance for your bird(s) -no Avian Vet or person elsewhere can say exactly what is right unless they weigh and monitor each individual bird regularly/often.