I have worked with the calculator a lot. I have found that if I put in the exact-precise data, it is very, very close or deadon. The centershot variable is very important. If your bow is listed in the list, that is what the maker says he cuts centershot too, but not every bow is the same on handmade bows.
You have to put in the variable for the exact bow and arrow you are evaluating. It works for me, and I do not use the personal form factor.
The spine calculator, just like an Easton arrow chart, is only a starting point to get close. You still have to tune your set-up with bare shaft. I have found it very accurate, much more than a chart, but I also shoot off an arrow rest and make sure I have no clearance fletching issues in tuning.
It sounds like you do not have the actual bow yet. I have been where you are. Wanting to be prepared. I have also found that to get the exactness of data that the calculator requires, I have to have the bow in my hands and shooting it. I need the exact weight at my draw with that bow. I need the exact draw length for me with that bow. I need the exact centershot. I measure my leather for strike plate with a pair of dial calipers, so I enter in the exact thickness. As long as I enter in the "exact" information, the tool has worked for me. When I first used it, I made assumptions about data. I have realized that will not work. It has to be exact and factual info you put in. It is a precise instrument that has been extremely helpful to me.