Amount of seed to feed budgie?

The retirement home where I work has a budgie. We took him to get his beak & nails trimmed & the lady said overgrown nails & beak is a sign of fatty liver disease & we are overfeeding him. We always just kept the cup filled but she said feed him once a day. So how much should he get? He has a huge seed cup in his cage that probably holds 1/4 cup or more. I've no idea how much he really needs. Probably way less. We give him raw fruit & veg pieces too & millet spray for a treat every few days. He's probably about 5 years old.