Dylan raises some good points. It's tough to wrap everything up in a bundle and tell someone "here's your efficiency." Plus, the definition of efficiency will not be the same for all organizations. An outbound center, for instance, is more concerned with producing as many leads/sales/surveys as possible whereas an inbound center is more concerned with fielding and servicing calls as quickly (efficiency) and thoroughly (quality) as possible. And let's not forget blended centers, where outbound and inbound calls occur dynamically. Obviously, the two cannot be measured by the same standard, so your Rule 75 system should be able to address this issue.
And efficiency can apply to various levels in a call center, such as software/hardware efficiency (voice/data sync, AM detects, abandons, etc.), agent efficiency (avg. talk length, wrap-up time, etc.), staffing efficiency (turn-over, seat occupancy, etc.), and sample/list efficiency (X amount of sample to produce X amount of results). If you're proposing that all these factors can be incorporated into a single number, I'm not sure how meaningful that metric will be. In my experience, you use thresholds to manage the daily variables of the call center (ie. minimum seats/hours to hit targets, min. sample/records to run full shift, min. staff scheduled to hit min. seats filled, etc.). But to combine all these thresholds into one number would mean very little to me. Prit, perhaps you've already taken this into consideration and it's a moot point by now. Just thought I'd throw in my 2 cents =)