The need to provide engineering solutions to full scale blasting problems has driven the development of several empirically based fragmentation models. Arguably, the most popular and successful models have been those applicable to surface blasting such as the Kuz-Ram model; and more recently the KCO model applying the Swebrec function. These models are based on what is generally described as the fragmentation associated with single hole firing conditions, that is, they do not consider the potential interaction of detonating blastholes on the breakage and fragmentation process. The application of precise and short delay timing using electronic detonators has demonstrated the need to incorporate modelling parameters that can adequately show the impact of timing on fragmentation. This paper describes an approach based on the hypothesis that a correction factor can be applied to the uniformity of fragmentation, the expected mean size of fragments and the proportion of fines generated during the blasting process. The correction factor is a function of the ratio between the interhole delay time dt and the minimum response time Tmin, which is defined as the time that elapses between explosive detonation and rock mass movement and is a function of explosive type, rock properties and blast design parameters. The use of this correction factor to predict relative changes in fragmentation from changes in interhole delay timing is demonstrated using a newly proposed two component Swebrec function. Preliminary data analysis demonstrates the practicality of the method. As with any empirical methodology, site specific calibration and further testing is recommended.