Daniel, I think you're writing off Stu's calculator a bit too soon. I've always found it very helpful, but needed to tweak the "personal form factor" a bit to get the predicted results to match up with actual practice.
Recently, I've revisited my calculations ( I save the ones that work well for me - have 2 or 3 for each bow).
I found that I have been guilty of the "garbage in - garbage out" syndrom. Couple of examples:
1. I've been using the "3 5-inch feathers" setting because that is what I use. BUT, I also add a cap wrap, so I weighed the bare shaft and then weighed it again after adding the 3 5-inch feathers and the cap wrap. Came out to 30 grains, so I selected "other" in the fletching block and entered 30 grains in the block below. Stiffened up the calculated dynamic shaft. If I were to put a 1/4" collar on the nock end, that would add some more weight to the nock end and stiffen things even more.
2. I've been entering 11 gr for the standard aluminum inserts because that is what Gold Tip had on their web site (actually, 11.4 gr). Today I weighed the inserts which came to an average weight of 15gr. I use hot melt glue which fills the grooves in the insert so I added another 1 gr. Now I am entering 16gr for the insert weight. Changed things a bit again and came much closer to what I'm seeing on the target.
I find when I put in the actual cut-from-center distance, correct string type info and proper arrow info, I no longer have to fudge the personal form factor.
Pretty long winded, but I will bet you if you have an arrow that flies really well from your bow, you can put the proper information for that arrow in the calculator. If it matches the dynamic power reading for your bow, that's fine. If not, you can change the personal form factor until it matches the dynamic calculation for that "good" setup. That calculated dynamic spine reading becomes your "gold standard".
Now, you can change arrow shaft types/spines/lengths and point weights to get a matching dynamic spine number and have a new arrow that will fly right along with the original arrow.
You're right about Stu being smarter than most of us in building that spread sheet. It took an enormous effort I'm sure, but that's his job as well as hobby. Sure glad he's a trad archer as well as a math/Excel genius. He's created, at least for me, one of the most useful tools I know of for the trad archer.