Network Calculus is a generic theory conceived to compute upper bounds on network traversal times (WCTT - Worst Case Traversal Time). This theory models traffic constraints and service contracts with arrival curves and service curves. As usual in modelling, the more realistic the model is, the more accurate the results are, however a detailed model implies large running times which may not be the best option at each stage of the design cycle. Sometimes, a trade-off must be found between result accuracy and computation time. This paper proposes a combined use of two simple class of curves in order to produce accurate results with a low computational complexity. Experiments are then conducted on a realistic AFDX case-study to benchmark the proposal against two existing approaches.