A previous paper by the authors (Stockton and Middle 1982) examined the conditions required for using the regression technique to develop costing models of acceptable estimating accuracy. This technical note (an extension of the earlier work) proposes the use of the Average Proportional Error (E) as an improved measure of indicating the estimating accuracy of models. A modified regression procedure is then described which enables the development of estimating models to be consistent with the method of measuring their accuracy. Finally it is shown how the relative manufacturing volumes of components can be allowed to influence the estimating accuracy of cost models.