QOS to give queue traffic for IP only if all users are idle

I have a PCQ qos setup going that equally shares available bandwidth. This seems to be working well. I would like to have the current QOS setup but would like to limit a set IP 192.168.1.52 to have very low priority. Basically it must be able to use the full bandwidth of 4meg when no other machines are using the network. As soon as anyone on the network starts using the internet I want the low priority machine to be basically crippled to zero traffic or as close to this as possible. So after this machine is crippled all other machines must use all available bandwidth equally as I have setup now.

Any ideas? Please be easy with me as I am very new to Mikrotik.

Thanks!

Anybody have any idea how to do this?