There's no useful measure of "free" ram, and used ram isn't necessarily total-free. "Free" really means unused, and there should be a minimal amount of unused ram. Unused ram is wasted ram.

Something more useful than free ram though, would be a measure of available ram. Ram that can be quickly made available without and paging. This would approximately be free+buffers+cached minus executable and library files in cache, though if a file like that is actually not used, then that would be available too.

In short: there's really not any good measure