The first time I did the flush I had cereal for the breakfast and jacket potato with fresh vegetables (no butter or oil). The food was fat free but still quite heavy. That's why I think I did't pass any stones. The second and third times I did flush I had just a glass of freshly sqeesed vegetable juice and a bowl of cereals with soya milk and I passed about 30 stones during 2nd & 3rd GB flushes( I was VERY hungry all day long though)
So I supppose you have to keep it very light and not eat ANYTHING after midday to develop a pressure on GB to push out stones.