Hi @fred4 and thanks for your query. A fixed point would be the top left of your screen which represents the value (0,0) of the Eye Gaze Data as reported by Tobii Core SDK and Stream Engine API.
If by Group Centre of users, you mean the screen location (x,y) that receives the most gaze? This would be more dependent on the *time* or fixation duration of the gaze rather than simply the ‘mean’ of gaze locations reported as data will still be sent if the user is not looking at screen but still trackable.
With the Tobii Core SDK, we demonstrate a few examples that demonstrate how to work with Fixation Data @ https://github.com/Tobii/CoreSDK/tree/master/samples/Streams with further information about fixations you can read @ https://tobii.github.io/CoreSDK/articles/streams.html
Please do bear in mind however that using a consumer-level Tobii Tracker (4C, EyeX) for anything other than interaction purposes requires the purchase of a special analytical licence. You can read more about this @ https://analyticaluse.tobii.com/
If you could kindly clarify your Tobii hardware model and corresponding SDK, we would be better placed to help you.
Please do let us know if we can provide any further information.