Not to be controversial but I think the stirrup position formula is debatable. How can there be a formula that is perfect for every rider? People have thin thighs, fat thighs, skinny calves, thick calves, varying degrees of flexibility in their joints, long torsos with short legs, long legs with short torsos, etc. Plus there are so many different disciplines, footings, types of terrain, etc.
I have three good friends who are all excellent riders. We ride English. All of them can stay on when horses rear or buck, and all of them can gallop in rough mountain country on a variety of horses. Each of us rides with our stirrup at a different length. My friend with a short torso and long legs prefers a very long, dressage leg and she uses this even when jumping cross country. I prefer a shorter leg than one friend who is my exact same height. Besides, what if you have long arms and short legs? When my horse was green I rode her with a shorter stirrup than I do now. I ride a choppier trot with a shorter stirrup than a smooth trot. I even ride my two horses with different stirrup lengths. :)
My point is: don't let anyone tell you what your stirrup length should be. Ride where you are most secure and comfortable. If I shorten up more my knees and ankles hurt. If I lengthen more I lose stability. Change your length during a ride if you are on a different horse and it feels wrong.
And when we were talking about leaning back vs. leaning forward: I have found that different horses seem to have different centers of gravity. Sometimes to get in balance with a horse I have to ride leaning forward more than I usually do. On one small mustang I even had to lean back a little to get in sync with her when she galloped. To me it makes sense since horses have different lengths of back, neck, legs, different girth lines, etc.