There are sometimes when you will have to forward all your proxy requests to a parent proxy, we maybe will have so many proxys on the network but what that hell.
After configure you squid proxy (I have a <a href="http://soad1982.blogspot.com/2009/07/squid-initial-set-up.html">quick start here</a>) you will just have to add a couple of lines to redirect the proxy settings.
I know squid could change sometimes the lines, so here is the version I've...