Hi Joost, and thanks for your query. There is for the time being no tools or support for calibrating directly within the Tobii XR SDK unity engine, although this option does exist for the Desktop SDK for Unity (with the purchase of a special licence to do so).
For users of the Tobii XR SDK, headset vendors provide a built-in way to calibrate which can be launched from the SteamVR dashboard in the case of the Vive Pro Eye. For Devkit users, one can download the Legacy version of the Mirrors Demo and run the setup part in order to calibrate.