I figured you were way weak half way through your post. Been there and done that. Carbons have had me scratching my head a few times unlike other shafts. I have been able to get a carbon to tune bare shaft when real weak. All that FOC I think. I also have some tuned now to my bow that are somewhat weak on Stu's calculator and others that are about spot on. Both shoot great. However, the weaker set-up is much less tolerant to point weight change or maybe more depended on how you look at it. What I find is a little change in brace height or point weight makes them not shoot as tight groups. The bare shaft flies to the right spot, but something is just off. Long range bare shaft can correct in flight after 20 yards or so. I can add weight and take it off of these arrows, and I don't get a noticeable move weak or stiff. I just get less tight groups.
I figure the quick reactions of carbon with the high FOC does something that can cause some kind of timing thing to work out for me with different set-ups if I hit the sweet spot. I think for me a lot of overhang on my shaft does something also with some kind of whipping effect, and when I hit the sweet spot it works.
What I have done now is move more in the direction of Stu's calculator, but with shorter arrows. I did this first because I had a pile of arrows with slight dings on one end or the other. I cut off a inch or more and got a safe shaft. I started playing with shafts no more than 1 1/2" longer and most more in the 3/4" - 1" longer than my draw. Then I loading up the end until they shoot well with a fine tune of a 1/4"-1/2" cut. Often I found I had a much shorter arrow, but same point weight or less point weight. I looked for a noticeable move from stiff to weak with point changes. No kind of different, but real noticeable shooting stiffer or weaker by adding or taking off 25 gr. or changing shaft length 1/4". Now my arrows fly better and more consistent with slight variables, and show clear weak or stiff with changes. My current arrows are:
GT trad 5575 30 1/4" 50 gr. brass and 175 point
GT trad 5575 29 3/4" 100 gr. brass and 125 point
Note that they are close on length and have the same point weight, but the second has a longer flexing shaft length by almost 1" due to shaft and insert length. The first is the one that shows distinct difference in point changes while the second is the one that can be vague on stiffness changes and looses accuracy with slight brace changes.