Normalization of D-instanton amplitudes

D-instanton amplitudes suffer from various infrared divergences associated with tachyonic or massless open string modes, leading to ambiguous contribution amplitudes. It has been shown previously that field theory can resolve these ambiguities and lead unambiguous expressions for contributions amplitudes, except an overall normalization constant remains undetermined. In this paper we show theory, together the world-sheet description of also fix constant. We apply our analysis special case two dimensional obtaining results in agreement matrix model obtained by Balthazar, Rodriguez Yin.