Hi @tungtwister, sounds like a super interesting project and thanks for letting us know. Indeed after asking further, we do not have minimum figures of accuracy as for head tracking this very much depends on numerous environmental and user specific conditions which will vary accordingly.
That being said, if you have checked out some of the head tracking samples as included with the Core SDK you should hopefully find the sensitivity more than adequate for your needs. There will always be some degree of ‘wiggle’ in head tracking even if trying to remain perfectly still, so you should be able to setup a buffer that will qualify as stillness.
Sorry we cannot provide more specific metrics, but any other questions, please don’t hesitate to ask.