In this paper we describe how a functional version of the 4-variable model can be decomposed to improve its practical application to industrial software verification problems. An example is then used to illustrate the limitations of the functional model and motivate a modest extension of the 4-variable model to an 8-variable relational model. The 8-variable model is designed to allow the system requirements to be specified as functions with input and output tolerance relations, as is typically done in practice. The goal is to create a relational method of specification and verification that models engineering intuition and hence is easy to use and understand.