The forecasted traffic growth makes the transmission capacity in backbone optical networks a scarce resource. Enabling the use of high-spectral-efficiency modulation formats (HSE-MFs) such as DQPSK or PM-QPSK for 40 or 100 Gbps respectively is one of the suggested on-going researches. However, some lightpaths may not require the highest rate and this call for mixed-line rate (MLR) networks. In MLR networks xQPSK formats' reachability decreases due to cross-phase-modulation (XPM) induced by neighbouring OOK channels. Therefore, to increase MLR networks spectral efficiency it should be able to use HSE-MF for high capacity demands. However, the choice of the MF to establish a lightpath is dependent on the length to satisfy the Bit-error-rate (BER) constraint. For that, we should route the high capacity demand in the shortest physical routes and vice versa. Thus, in this work we demonstrate the importance of traffic serving ordering based either on demand capacity or physical route in the static phase of the network design. We use the Core-Topology of the Pan-European network with a static traffic matrix of Log-Normal distribution. Serving traffic in the increasing order of the demands' shortest-path shows the best performance in term of blocking and resources utilization