In recent time book-ahead (BA) reservation has drawn increasing attention as multimedia and distributed applications become functionally richer. Book-ahead reservation has proved itself as an effective technique for time sensitive applications that require high amount of bandwidth with guaranteed QoS. One of the major concerns in the implementation of BA reservation in a QoS-enabled network is the preemption of on-going instantaneous requests (IR) calls. Preemption is a threat to service continuity which is seen as an important issue from user's perceived QoS definition found in recent studies. Earlier models proposed to reduce IR preemption use constant look-ahead time. This paper proposes a novel model to calculate look-ahead time dynamically taking traffic parameters and network state into consideration. Simulation results show that the proposed model achieves lower preemption probability, higher throughput, lower wasted throughput and higher effective revenue.