A simple continuum mechanics model relating the quality index,* Q, to parameters of the deformation curve of alloys A356/357 is presented. The model incorporates the limiting effect of particle cracking on the ductility on a phenomenological base and it is used to predict the behaviour of the quality index as the material is strengthened by either ageing or increased content of Mg. The analysis shows that for every temper there is a matrix strength that maximises the Q-value. For strong alloys the model suggests that peak ageing can reduce Q. This effect is stronger when the ductility of the material is low. The possibility of using the quality index to optimise the combination of the alloy chemical composition and temper is discussed.