Ultimately, matching the dynamic poundage of your bow to the dynamic spine of the arrow seems pretty reliable.
Using the calculator to get that, however, opens up lots of room for error. Especially, as bulldog said, if your bow is not listed. Nor does it have any way of telling one when it's a form issue rather than a spine issue.
I do better, with less frustration, using the old reliable rules of thumb for spine, and trusting my eyes seeing the arrow flight.
It can be informative playing with the calculator to see how changes in your setup affect spine, at least in numbers.