This paper deals with design of reference battery state-of-charge (SoC) trajectory for an energy management control strategy of a parallel plug-in hybrid electric vehicle (PHEV) in the presence of varying road grade. In order to analyse optimal behaviour of PHEV powertrain and optimal SoC trajectories in the blended operating regime for different road grade profiles, a backward-looking model of a PHEV bus of parallel powertrain configuration is considered and dynamic programming (DP) is used to optimise powertrain control variables. Based on insights gained through analysis of the DP optimisation results, a novel approach for synthesis of reference SoC trajectory for BLND regime and non-zero road grade case is proposed. The effectiveness of the proposed synthesis method is demonstrated by simulations of PHEV bus over different driving cycles and road grade profiles, where the corresponding synthesized SoC trajectories are used as references meant to be followed by realistic control strategy. Finally, the obtained simulation results are verified against the DP benchmark in terms of total fuel consumption.