Hi tom,
my first advice to you would be not to do any calibration at all, and see how much error do you get. If the error is not significant, then you might not even need to calibrate, or you can compensate for the error in software (if it’s consistent enough). From my experience, this usually works well if the subject doesn’t wear any aid.
I played with different eye trackers for this purpose and we can discuss this topic further, but let’s move it somewhere else because it’s outside the scope of this forum. You can drop me a contact, or propose some other channel for communication.
cheers,
pablo