I've done some testing with Stu's new calculator since Christmas on wood, carbon, and aluminum arrows. And I agree with Blackstick.....whether you enter your own generic specs or use the preset ones....the center cut measurement is critical to get accurate. Garbage in/garbage out.
My own experience with this calculator with two generic longbows, is when I find an arrow that is within two pounds of the bow's spine spec it is usually borderline stiff. I will shoot to test and have always ended up front loading that arrow with point/insert weight until it shows approx. 5# weak on the calculator. I end up with a great flying, impact at point of aim, higher FOC arrow.
Just my experience.....but I'm glad to have that calculator from Stu....sure has saved me $$$ trying to figure out what I need.