There are compelling reasons for a Controlled Natural Language of isiZulu in software applications, which requires pluralising nouns. Only ‘canonical’ singular/plural pairs exist, however, which are insufficient for computational use of isiZulu. Starting from these rules, we take an experimental approach as virtuous spiral to refine the rules by repeatedly testing two test sets against successive versions of refined rules for pluralisation. This resulted in the elucidation of additional pluralisation rules not included in typical isiZulu textbooks and grammar resources and motivated design choices for algorithm development. We assessed the potential for reuse of the approach and the type of deviations with Runyankore, which demonstrated encouraging results.