Molecular Communication via Diffusion (MCvD) aims to establish communication links at the nanoscale by utilizing chemical signals for information transmission. After encoding the bits in a physical property of a molecular wave, the transmitter releases the molecules into the fluid environment where they diffuse randomly according to the laws of Brownian motion. For consequent bit transmission scenarios, the random nature of the channel causes inter-symbol interference (ISI) for the system, which affects communication performance and poses limits on the range and the bit rate of the communication. Due to facing different amounts of ISI for different bit sequences, each sequence combination should be evaluated when deriving the theoretical bit error rate of an MCvD system. Emphasizing the fact that some bit sequences produce considerably more bit errors than others, this paper aims to reduce the number of evaluations required to derive theoretical approximate bit error rates while preserving sufficient precision.