Perhaps looking at this topic from a different vantage point would offer better perspective for some?
Coming from the proper alignment camp, consider tibia angle when the skier is standing cuff neutral, not pressing forward or back on cuff. This angle is critical to balance and the ability to pressure the ski shovels before the fall line or top of the turn. Before we get to this point we must first assess the skier's dorsiflexion range of motion and adjust the ramp angle and cuff forward lean to create a good net forward lean angle for the dorsiflexion needs. Once this is done, we do not go back and change forward cuff lean to change knee position over the ski. In order to now change where the knee plumbs over the ski we need to change the delta angle, created by the differential in height between the toe AFD and the heel piece. Here we are able to change the lower leg angle by changing these heights with lifts under the binding or lifter plates screwed to the boot soles. My personal opinion and method is to see the knee plumb right over the tip of the boot clog when standing cuff neutral clicked into the bindings. This is my static target but realize this is not an exact science just a good starting point. Then we need to ski it dynamically as the body will seek to find equilibrium while in motion and this can not be cheated. We play with shims to zero in then make permanent modifications.
Now, getting back to skiing and boot flex. Once we find and set the optimum angles on the sagittal plane the skier should feel a relatively tall stance with slight tongue pressure into their shins. The skier should feel comfortable making fuller extention into the turn reducing the angle between the femurs and torso. They should feel the support of the tongues at the top of their turns and that this instills confidence to more fully extend gaining a leverage advantage over the boot flex. From this position the skier should be able to crush the boot driving energy to the ski shovels through the fall line. The skier will also discover that one legged skiing becomes much much easier because they able to move into and rest against the support of the boot tongue by moving the hips ahead of the foot. Conversely if the boot cuff angle is too far forward the skier will have to move their hips back to balance and be unable to steer the ski effectively.
A boot set up with too much lower leg angle will not permit a fuller extension and not permit the skier to pressure the shovels before the fall line. This skier will remain in a more flexed position and tend to ski more mid to rear foot pressure. The lower leg angle will be excessive while the spine angle will be more vertical.
A boot set up with too little lower leg angle will not allow the skier to get the leverage advantage to flex the boot. This skier will have to compensate by breaking over at the waist to find balance as they will have difficulty flexing the boot.
Finding this optimum angle is probably the most critical equipment factor to figure out and get correct! This can make the difference whether a skier is able to use a stiffer flexing boot or not. It will also determine if the skier will be able to shape their turns well before the fall line. Get this right and you will appreciate stiffer boots.
I have done extensive testing with the sagittal plane adjustments with instructors and myself and these have been my findings. When you see your students or friends skiing and notice the spine angle and the shin angles are not pretty much parallel, one issue to consider is their fore/aft equipment alignment. If they are bent forward at the waist and the lower legs are more vertical or you see their knees flexed way over their toe pieces and the spine more vertical, suspect sagittal plane alignment issues.
The skier in the video above looks aligned pretty well on the sagittal plane with his equipment!