According to Google the link tag containing the URL to the pubsubhubbub hub (This is a really terrible name) needs to go under the Atom entry element like so:
<?xml version="1.0" encoding="utf-8"?>
<feed xml:lang="en-GB" xmlns="http://www.w3.org/2005/Atom">
<!-- ... -->
<entry>
<link rel="hub" href="https://pubsubhubbub.appspot.com/" />
<!-- ... -->
</entry>
</feed>
However, all the implementations and blog posts I have seen add it to the feed element instead like so:
<?xml version="1.0" encoding="utf-8"?>
<feed xml:lang="en-GB" xmlns="http://www.w3.org/2005/Atom">
<!-- ... -->
<link rel="hub" href="https://pubsubhubbub.appspot.com/" />
<entry>
<!-- ... -->
</entry>
</feed>
What is the correct location for it?
It should be at the feed level: http://pubsubhubbub.github.io/PubSubHubbub/pubsubhubbub-core-0.3.html#discovery (that is btw. the same spec, Google is referencing).