Electronic level alignment at the interface between an adsorbed molecular layer and a semiconducting substrate determines the activity and efficiency of many photocatalytic and photovoltaic materials. Accurately describing this interfacial level alignment requires a proper description of the anisotropic screening at the interface. This necessitates the use of many-body quasiparticle (QP) GW techniques for the level alignment, and solving the Bethe-Salpeter equation (BSE) to obtain accurate absorption spectra. We compare the calculated and measured interfacial level alignment for prototypical photocatalytic (water and methanol) and photovoltaic (catechol) layers on rutile TiO₂(110). Specifically, we compare the interfacial level alignment from GGA DFT, hybrid DFT and G₀W₀, scQPGW1, scQPGW₀, and scQPGW quasiparticle calculations with two-photon photemission (2PP), ultraviolet photoemession spectroscopy (UPS) and inverse photoemission spectroscopy (IPES). These demonstrate that a QP GW treatment is required to reproduce the measured interfacial level alignment. Furthermore, we find the quasiparticle energy shifts Δ are linearly dependent on the fraction of the wave function density within the molecular layer f_mol and the bulk substrate f_bulk. For the unoccupied states, the same correlation holds for all the molecular layers studied. This allows one to describe the quasiparticle energy shifts semi-quantitatively for larger molecular layers on TiO₂(110) based on more tractable DFT calculations.