While the resolution of the webcam is (obviously) important when discerning accuracy, I feel like you may be conflating two terms here. Specifically, going from the resolution of the webcam to an estimate of so many pixels of accuracy using basic trigonometry will necessarily depend on the method you're using to convert the webcam input into eye-tracking data.

The <70 pixel figure for GazeHawk's accuracy is based on testing against real, labeled training data. That is the distance, on the screen, by which our calculated gazepoint differs from the true location at which the user's gaze was directed. It is only loosely correlated with webcam resolution, in that a higher webcam resolution corresponds to a larger pipeline - more input pixels being dumped into the eye tracking algorithm. I could be wrong, but it sounds like you're discussing the size of the eyes in the input image.

Also, at this point a discussion of accuracy vs. precision becomes germane. The use of higher resolution video as an input can often impact one but not the other.