Yeah, I’m supporting both the Vive and Rift. They behave a little differently.
Rift: Wherever you put the camera on the character, that’s where you put the HMD. I place it on my characters shoulders, so if the center of the character is [0,0,0], then his eye position is at [0,0,70]. I store this value in an “HMD Type Offset” vector.
Vive: This is based off of the very bottom of the characters feet, on the floor. So when the player picks up the vive HMD and puts it on their head, they move the camera to their own eye height, which should match up with the characters eye height. If the Vive is being used, I set my “HMD Type Offset” value to [0,0,-90], and then calibrate to the players height.