This study aims to improve synthesis quality using the Holmes, Mattingly and Shearme (1964) phonetic-level synthesis-by-rule (SbR) method, both by increasing the inventory of allophone segments and by automatically optimizing the values of the segment-table entries. Every occurrence of each phoneme is first optimized using a separate segment model. Initial estimates are iteratively refined using an analysis-by-synthesis procedure based on comparisons between the natural and rule-synthesized speech spectra, so imposing the inherent continuity constraints of the SbR model. The paper describes this automatic process, whose output is a set of individual segment tables for high quality segmental copy synthesis. These individual tables will later be combined to form allophone models for improved synthesis by rule.