I completely agree with you that it is probably impossible to compute (per my original comment). But at least understanding what the target is helps imo.
Using metrics to gauge how well the enterprise is doing on the whole and to influence policy decisions and strategy is absolutely good and desirable. It's only when metrics become rules or quotas or measures to compare people or departments as a policy that things go bad.
I completely agree with you that it is probably impossible to compute (per my original comment). But at least understanding what the target is helps imo.