Total legal spending as a percentage of revenue (TLS/Rev) leads the metrics pack in terms of importance. I have written an article about some aspects of that figure.
and many posts after its publication (See my post of Dec. 3, 2007: reasons why TLS/Rev declines, with 11 references; Dec. 5, 2007: stability of the ratio over a decade; Feb. 6, 2008: total legal spending declines as a proportion of revenue as revenue increases; Feb. 17, 2008 #1: supporting data from ACC survey; July 20, 2008: comparative data over four years on TLS/Rev; and Dec. 19, 2007: pattern holds within an industry.).
Other drivers may influence the trend of TLS/Rev down as revenue trends up, such as client satisfaction or what leads to it, invention activity within the company, pay of in-house counsel, and the ethical stance of companies (See my post of Aug. 4, 2008: unknown relationship to client satisfaction; Aug. 13, 2008: relation to R&D spend; March 13, 2008: compensation rises but not total legal spending; and June 26, 2008: do ethical companies spend less on their legal budgets.).