These tees are easy to use. Simply push it into the ground until the wider part of the tee is flush with the ground. Your tee height will never vary. The tees seem to be reasonably robust. I played a round which included 15 drives (played 3 off the tee on one hole) all with the same tee and it is still in one piece. The tee lengths quoted are the total length of the tee, not the tee height. I bought a pack of 70mm which measure 68mm and give a tee height of 50mm, and a pack of 51mm tees, which give a tee height of 32mm.
Now I no longer need to feel guilty when I lose a tee, because they are biodegradable, eventually.