Top-up after DBF but not overfeeding

Hi mommies, i am a first-time mommy.. my baby is 1 mo++ now and i am having a challenges on getting him on dbf.. every time after nursing, he seems unsettled and (sometimes) crying as it is not enough for him.. if we do top-up from every feeding, how to know and rest assured that he is not overfed? Most of the time, he spill / vomit after feeding.. Thank you mommies..