The Z and Z2 have 5 buttons with deep click. However these five buttons rely on just two internal force sensors; one on of each side of the mouse.
So when you push the main left mouse button, or the left fingertip button, the left force sensor is used. While the right force sensor is used for right click and the right fingertip button. Individual calibrations are applied to these four buttons to ensure each can access the full 0-100% range. The middle button is different however.
When you press the the middle button pressure is detected by both the left and right force sensors. The side that registers more force depends on the angle at
which you press the scroll-wheel. Push it down and to the right, and the right sensor will detect more force than the left, and vice versa. This means that unlike the other four deep click buttons the middle button cannot rely on a single set of calibration points. So instead it uses two.
While the middle button is pressed it checks both the left and right force sensors and selects the sensor with a higher force reading. That force reading then has the calibration from the fingertip button of that side of the mouse applied to it giving the final percentage value.
As described above the middle mouse buttons effective calibration is dependent on the calibration of both the left and right fingertip buttons. So while it cannot be calibrated directly, it's calibration can be improved by adjusting the calibration of both fingertip buttons.
This unfortunately is not a perfect solution. So you my may not always get the full 0-100% range on the middle button. But when both fingertip buttons are well calibrated, the middle button should get enough usable range that its deep click inputs can be adjusted to suit.
For example if you can only consistently access 30-100% on the middle button, just set your your deep click inputs to activate somewhere within that range.