There is a lot of posts about the Free memory on Ubuntu.
Mainly the commands free or cat /proc/meminfo
MemTotal:         960176 kB
MemFree:           55292 kB
Buffers:           91188 kB
Cached:           526496 kB
SwapCached:         5912 kB
Active:           410628 kB
Inactive:         423128 kB
However, I cannot find a definitive answer as what is the actual usable memory. Ubuntu reserves some of the memory for the Buffers and Caches. And may release some if a greedy process asks for it.

So, what is the "reasonable" usable amount of memory?

Is "Cached" or "Inactive" a fair indication of what could be used if required (minus a %) ?