Where is the best position of the chat/help floating button in a content-heavy web page ? Is there any best practices to consider when it comes to eye tracking?
It depends. How important is this feature for your audience?
Most websites place their live-chat at the right bottom above the fold (i have no numbers for this, its just my personal perception), but if the live-chat is really important then you could also place it in the header (for example on the right side next to the navigation with a call to action button).
If you don't have a live-chat right now you can make your users aware of this new feature by giving the floating button a subtle animation, for example a small shaking when the users visits your site and see's the button for the first time.