Runtime.freeMemeory() returns an approximation to the total amount of memory currently available for future allocated objects, measured in bytes.
Runtime.totalMemory() returns the total amount of memory in the Java virtual machine. The value returned by this method may vary over time, depending on the host environment.
Runtime.maxMemory() is proabaly the only 'fixed' value.
As I see it, these values correlate somewhat loosly and there is no formula for it that would work all the time (if ever).