Hi -
My post response is way too late, but I had been researching this value for MPE 2.1 and found a good reference in the CUCM SRND guide - this guide is becoming one of my main go-to references for best practice recommendations. Full link is here - http://www.cisco.com/en/US/docs/voice_ip_comm/cucm/srnd/7x/uc7xsrnd.pdf
Regards, Ginger
System Resource Unit (SRU)
Use of the G.729a codec and high-bandwidth videoconferencing (above 384 kbps) will affect overall system capacity. Estimating the overall system capacity for Unified MPE involves the concept of a system resource unit (SRU). The number of SRUs available to a system is based on the type of media convergence server (MCS) deployed and is reported on the Meeting Configuration page in the Web Administration Center, given a particular codec and video configuration. Each voice, video, or web SRU required for a user session consumes a corresponding voice, video, or web user license (UL) in the system.
A video session with a bit rate of 384 kbps or less requires one SRU. Two SRUs are required for any video session greater than 384 kbps and up to 768 kbps. A voice session with a G.711 codec requires one SRU, and a voice session with a G.729a codec requires five audio SRUs. Web conferencing requires two SRUs per user session.
When a participant joins a video conference with a video bit rate of 384 kbps (or less) using the G.711 codec, Unified MPE allocates two SRUs, one video user license (UL), and one audio UL for this participant. If this same video endpoint joins with a G.729a codec, then six SRUs are allocated for the voice and video portion of the call.
Unified MPE does not take the flow-controlled bit rate into account for its system capacity calculation. For example, if a 384 kbps participant joins a conference with a video-session bit rate of 768 kbps, that participant lowers the session bit rate to 384 kbps, but Unified MPE still holds two video ULs for the existing session.
Note 
The Solution Expert Tool can assist in determining the correct MCS model size based on your design criteria. This tool is available (with appropriate login authentication) at http://www.cisco.com/go/sx.
