In the search for next-generation optoelectronic devices, there has been a growing interest in donor-acceptor (D-A) materials, including D-A co-crystals and D-A copolymers, for their application in organic solar cells and organic lightemitting diodes. D-A complexes, by definition, exhibit charge transfer (CT) in their ground and select excited states. In the search for structure-function relationships to integrate into high-throughput screening and machine learning protocols, several studies have assessed ways to predict the degree of CT (DCT), or ionicity parameter, in the S0 state of D-A materials from molecular quantities such as orbital energies, vibrational frequencies, and geometric parameters. Early evidence shows a relationship between DCT in S0 to the magnitude of effective CT integrals, commonly used in models of charge transport. The DCT in the first electronically excited state of D-A dimers, S1, has emerged as a key quantity for predicting radiative and non-radiative lifetimes in D-A materials, including intersystem crossing rates and fluorescence lifetimes. These lifetimes are particularly difficult to compute directly using D-A dimer models, as energy levels and transition dipoles ofter differ substantially between the molecular cluster and material. A variety of DCT metrics for excited-state calculations have been put forward, as reviewed recently. We construct a similarity metric, η, as follows. First, define a molecular orbital