I would incorporate Component ERA (i.e. CERA or ERC, as it is sometimes known) into your calculation. I think CERA is one of the overlooked gem stats of pitcher evaluation, especially for relievers.

A reliever who gives up hits and/or walks and allows inherited runners to score (as opposed to getting an out and allowing an inherited runner to score) has done a lousy job, IMO. Yet, scoring rules are such that none of the resposibility for the run scored is reflected in the reliever's ERA. With CERA it is.

DZ