Hi Julio,
I think that you can get an accuracy in the 3 cm range without individual calibration for most users. But to be honest: I don’t have any data to support that claim, only my own experience with the product. I’d recommend that you start out without calibration and see if the accuracy is good enough that way. If not, you could consider a simplified two-point or five-point calibration instead of a full-blown nine-point.
The EyeX SDK doesn’t support custom calibration, but the Gaze SDK does, and it is possible to use both in the same application.