I have VRIK set up and per your advice I used the following line to calibrate height:
float sizeF = (ik.solver.spine.headTarget.position.y - ik.references.root.position.y) / (ik.references.head.position.y - ik.references.root.position.y);
ik.references.root.localScale *= sizeF;
My problem is that my model (avatar)'s face is right in front of the user when placing the HMD on his head and it's hiding the scene. Would it make sense to offset the head in some way? and if so how?
I also have another problem where my model is toptoeing after using the adjustment above. Should I just add some constant to the y position? it doesn't feel very robust doing that.