Coulomb interactions govern most optoelectronic properties in semiconductors including bandgap, excitons, polaron formation, light absorption, interaction of carriers with defects, charge and energy transport. A clear signature of the strength of many-body Coulomb interactions is the exciton binding energy (Eb). As such, the importance of Eb is perhaps second only to the fundamental band gap (Eg) in understanding the physics and efficiency of low-dimensional semiconductors. However, despite their importance, Eg and Eb are difficult to measure. Here, by choosing 2D halide perovskites as the material and electroabsorption as the experimental probe, we measure Eg and Eb with a precision that is an order of magnitude better than typical methods. The Eb values are both lower than previous literature reports and lower than expected from standard theory. Using dielectric spectroscopy, density functional theory, and quantum mechanical modeling, we demonstrate these low Eb values are a consequence of unique screening effects, such as superlattice screening and phonon screening. We demonstrate a clear correlation between Eb and Eg and provide design principles in order to a priori tune Eg and Eb to their optimal values. As such, this work lays the blueprint for Eg-Eb engineering of low-dimensional semiconductors, as an even more useful replacement to simply band gap engineering.