The accuracy of large-eddy simulations is limited, among others, by the quality of the subgrid parameterisation and the numerical contamination of the smaller retained flow-structures. We review the effects of discretisation and modelling errors from two different perspectives. First, we review a database-approach to assess the total simulation error and its numerical and modelling contributions. The interaction between the different sources of error in the kinetic energy is shown to lead to their partial cancellation. An optimal refinement strategy for given subgrid model, given discretisation method and given flow conditions is identified, leading to minimal total simulation error at given computational cost. We provide full detail for ho...