Mysterious 30ml

So I chanced upon a YouTube video about this 'mysterious 30ml' whereby babies leaving the extra 30ml, always at 30ml of leftover milk (mine happens 7 out of 8 feeds a day ?) For example, when I make 90ml of milk, she will consume 60ml (left over 30ml), if I make 120ml of milk, she will consume 90ml (left over 30ml too) Since she always leftover 30ml, I've decided to make 120ml during all her meals so she will consume sufficient of 90ml in a single feeding ? Any mummy experienced this?