This paper aims at minimising the video downstream bit rate of immersive video conferencing (IVC) applications by judiciously modifying the video quality based on the relative virtual positions of participants in the virtual environment. The paper reports on the results of a user study to assess the influence of participants' perspectives on the perceptual impact of relevant video parameters, such as resolution and frame rate. A mathematical model for video rate is proposed that expresses the total rate as the product of spatial resolution and frame rate. Results from the user study are combined with the proposed model to predict the rate parameters which will result in perceptually acceptable quality for a given user perspective. The simulation results show that by exploiting the proposed method, the downstream network load for the client can be significantly reduced with little or no impact on the perceived quality.