Hi @jaay, I can tell you that the gaze accuracy is a nominal 0.6 degrees assuming high quality calibration and well placed within the tracking area. Whilst not explicity stated for the C4, there is a whitepaper you can read on the accuracy of the EyeX which is somewhat similar (tho inferior in many ways) to the C4 @ http://link.springer.com/article/10.3758/s13428-016-0762-9
There is for the time being, no established quantified metrics for head angle tolerance.