We have set up guidelines/standards for scope, schedule, quality, and budget for our PM's to follow as to if they are 'red', 'yellow', or 'green' and not relying on a single calculation. We tried a calculation for a while, but found it lacked the 'gut feel' a PM may have. But we do have a minor issue of managers changing a PM's status on a project. That is more of a cultural issues we are currently attending to.