Hi Jacob, thanks for checking it out. Regarding the calibration step for front facing mode, I'm glad you brought this up. I did think of this, because the distance from the camera/screen to the hand affect the movement so much (the part where the angle of the hand is part of the position calculation).
And you are absolutely right regarding its use for the correct scale. For my implementation, I actually just hardcoded the calibration values, based on where I want the boundaries for the Z axis. This value I got from the reading, so in a way it's like a manual calibration. :D
But having calibration is definitely the right idea, I just didn't want to overcomplicate things at that time.
BTW, I am a happy user of Exponent, thanks for making it! I am doing some courses and also peer mocks for interview prep!