To answer your question, the amount of "spline" after the shaft has been extended is 100%
The inner shaft is peened on the steering wheel side, so that it cannot be slid out of the outer shaft completely.
The four nylon "rivets" everyone talks about are only nylon beads on the outer shaft. They have zero penetration or even dimples on the inner shaft. Their sole purpose is to provide a bit of resistance on the inner.
I have dissected these columns, and can guarantee with utter and complete certainty that there is absolutely no risk to man or beast associated with extending a P/S column to meet a manual box