The Calibrate function will move your hydra position origin to the center of the centroid of your two controllers minus whatever offset you have. For the third person character the shoulder midpoint is roughly 40cm above the actor origin so that’s the default for the offset. This means the calibration will move the new center from the hydra origin via shoulder midpoint to your actor origin. If you want to use this method with another centroid, for example, calibrate to where your headset is, you would make the additional offset a vector pointing from your actor origin to your camera, and then calibrate by having your hydras on either side of your head and calling calibrate. Note that if you use the T-pose you can also extract the length of your arms by grabbing the length of half the vector between the two points, which you can use to scale input to handle outlier arm cases.
If you want to use the manual calibration method, you call Set Base Offset instead. There is no centroid for this function so you need to figure out what offset you need to move the base so that you get roughly 1:1 motion. Note that this is typically the negative vector of where you want the origin of your input to be. Additional methods and notes on calibration can be found in the hydra thread starting at page 3. There are other calibration methods commonly used such as using a known point e.g. headset or virtual hands you ask people to match. This would be the function to use with those methods.
Always remember that the hydra base can be off-center, but not off-rotation. The wires should always come out of the back of the hydra (if it is off-center move it parallel to the center position, it will still be facing your back wall, not at an angle facing away from you). The reason for this is that the hydra uses hemisphere tracking and would require additional calibration if arbitrary rotation was allowed (and it’s not optimized for it).
Additionally I’ve finally managed to get Leap+Hydra auto-calibration to work, and it’s awesome. More on that later