Some companies have within their ambit business operations that are widely dissimilar. Conglomerates do, as do others, such as Alcoa, Altria, and universal banks. It is impossible for their law departments to find enough other companies to benchmark against that have a similar mix of business segments. The solution is segment benchmarking.
For example, if you are a manufacturer with a major financing arm, you can invite law departments in companies of either of those segments to participate. You have to figure out how to allocate some of the shared services lawyers and costs, but you can at least develop some metrics that give directional guidance.