If you glance down at Tobii 4c, you should be able to clearly see all the visible red lights: left, center, and right. If you can’t, the Tobii can’t see you very well either!
It can be somewhat useful to temporarily load the official Tobii software on Windows, if that’s an option for you. The Tobii software can give you a good feel for proper positioning with the Tobii 4c.
Be sure to recalibrate after significantly adjusting your position, adjusting the Tobii’s position, or if the tracking feels more inaccurate than usual
Lighting matters quite a bit. The Tobii 4c works better in a lit room. You also don’t want e.g. sunlight directly beating down on the device
Some folks feel the largest useable monitor is 24”, though Tobii claims 27”.
Try both the “control mouse” and “zoom mouse” modes out to see which works best for you. Demo here